CONCLUSIONS.

In reviewing the genera and species enumerated above, as to their number in which the structure of the macula has been observed, we arrive at the following results.

[42]To these have here been added two Bohemian species P. bohemicus and P. spinosus omitted above at page 64.

We have thus 136 species of 39 genera in which hypostomic maculæ have been found and only 36 species, in which it has been possible to study the structure of the maculæ through sections.

Common for a great number of maculæ in various groups, whether they show any organic structure or not, is the excessive thinness of their shell in comparison with that of the surrounding hypostoma. This is also in accordance with the tenuity of the cephalic eyes in relation to the test of the cheeks.

The structure which characterizes the macula as a visual organ, although often so rudimentary, is not in all instances spread through the whole substance of the macula. This structure has in several species been restricted to a narrow circumference of the body of the macula, to its interior apex as in Bronteus and Proetus. Only in the Asaphidæ, in Illænus and Lichas the entire macula shows this structure. Perhaps, to judge by certain indications in Bronteus, once in a larval or preceding stage of evolution the whole surface of the macula was also in that genus covered with lenses, which have been reduced.

The different groups in which the examined genera may be divided are the following seven, showing the remarkably great diversity of these organs. There is even in the same genus so great a variability that species with structure in the macula occur along with those devoid of any structure or also, as in Lichas, with a different structure. It must, however, be remembered, that the species of such genera may not be coeval.

Group 1.In the sectioned macula there is no trace whatever of any structure. The test of the macula considerably thinner than that of the hypostoma. According to the affinities of the genera they may be subdivided as follows.aBumastus, Dysplanus.bNileus, Symphysurus, ? Ogygia.cCalymmene, Homalonotus.dChirurus pro parte.

Group 2.The whole macula is of a spongious or irregularly polyedric structure, showing prisms in vertical sections. Its concordance with the structure of the deteriorated cephalic eyes or with the so called border zone is complete. If a supposition may be hazarded, I think that the spongious or reticulate structure in the maculæ is their real and original state, a lower stage of development of the visual organs, out of which the prisms in the cephalic eyes have been formed, and that the prismatic lenses in their decay, as seen in the Asaphidæ, reveal to us their original state and structure, and thus, as it were, return to the primary stage in the maculæ.

Asaphus, Isotelus, Megalaspis, Ptychopyge, Niobe, ? Megalaspides, ? Barrandia.

Illænus, Lichas, Trochurus, ? Platymetopus.

Encrinurus also probably belongs to this group.

Group 3.Maculæ with well developed globular lenses on the interior third of the macula. The blank part of the macula without any structure. In one instance (Chirurus spinulosus) the whole macula covered with lenses.

Bronteus, Chirurus spinulosus.

Group 4.The macula form sunk pits with smooth bottom. Structure unknown. In a few species there seems to be no less than four maculæ in two pair above each other.

Dalmanites.

Group 6.The elongate straight maculæ carrying on their innermost point from three to five diminutive, segregate ocelli situated on a clear, white patch. Proetus.

As yet these ocelli have not been observed on the related genera Cyphaspis, Phaëtonides, Phillipsia and Griffithides, but they may be preliminarily ranked here in consequence of their close conformity.

Group 7.The maculæ have in this group been reduced to a pair of tiny ocelli situated high up on the hypostoma, near its anterior margin.

Phacops, Acaste.

The position of the hypostomic eyes on the ventral surface of the trilobites is not quite so abnormous, nor so isolated a feature amongst the crustaceans as might at first be supposed. Amongst the recent crustacea there is probably none, which as to the conformation of its labrum or hypostoma resembles the trilobites so closely as the species of the genus Apus, however different they may else be in all other respects. We have studied chiefly the hypostoma of Apus cancriformis. In its shape as seen from the outside and as to its outlines it reminds strongly of the trilobite hypostoma. It is nearly square (pl. VI figs. 46, 47) with rounded corners. The anterior margin is arched and in the middle it has a broad tonguelike prolongation. On the sides of the anterior margin there are two short, sharply pointed wings. The lateral margins are thick, almost as doubled, and bent forward so that a groove is formed alongside them and the large, evenly vaulted median surface of the hypostoma. These grooves are continued in a short groove parallel with the posterior margin. From this margin a narrow oblique surface is slanting backwards, as it were, doubling that margin. On its surface there are two small, resplendent, white or clear spots, almost of the shape of a crescent, though more irregular when seen in higher enlargement (pl. VI fig. 49). They are translucent as to be visible on the interior side of the hypostoma. In Apus glacialis there are also similar spots in the same position, but they form short narrow, straight stripes, parallel with the border of the posterior margin. I have no opinion as to the nature or function of these white spots. I do not think that they at all are homologous with the macula; of the trilobites. On the other hand we see a little higher up on the exterior side of the hypostoma two faintly elevated elliptic tubercles which as to their position and shape highly remind of the maculæ of the trilobites. They are hollow on the inside and we have not been able to find any peculiar structure in them. That they have any visual function is precluded through the circumstance that the whole exterior surface is densely covered with microscopic bristles (pl. VI fig. 49) which are especially remarkable along the posterior margin.

In the class of the Cirrhipedia a sufficient number of facts is known to ascertain us of the presence of eye spots on the hypostomas of the larva. The researches ofSpence Bate[43]show in the larva of Balanus balanoides, on the hypostoma or labrum, which he calls proboscis, an eye, a little from the posterior margin of the labrum. It seems during the various moults of the animal to have changed its place, being in the two first moults situated near the anterior margin of the hypostoma. In Clitia Strömi it lies at the anterior part of the labrum (figs. 9, 10).Spence Batedoes not accept the opinion of their nature as eyes, but the following authors asDarwinandClaparèdeagree in considering these spots as eyes.Darwin[44]describes two such eyes in Lepas australis and, if I catch his meaning rightly, one on each side of the labrum.Claparèdehas given a figure of the larva of Lepas anatifera[45]in the hypostoma of which (by himcalled »Rüssell», proboscis) a single eye is seen, having at the sides two round problematic organs, that he at first considered as auditory cells, but later as eyes.Hesse[46]has also in a species of Scalpellum seen several coloured spots on the inferior surface of the carapace, below the antennæ and he expressly says that they are eyes and a special pair of supplemental eyes that are becoming functional after the first moult.

[43]On the development of the Cirripedia, Ann. Mag. Nat. list. 1851, p. 324, pl. VI figs. 1, 2, 3, pl. VII fig. 10.

[44]Monogr. of Cirripedia I p. 17.

[45]Beobachtungen ... über wirbellose Thiere, 1863, pl. XVII fig. 22.

[46]Mémoire sur les métamorphoses que subissent les Anatifes appelés Scalpels obliques. Annales des Sciences nat. 1859. Tome XI p. 163.

These sternal eyes in the larva are previous to the eyes in the dorsal side of the later stages and whenever such are discovered in other groups of adult animals as the trilobites they must of course be considered as remnants from the larval stage. As a rule, in several tribes of the crustacea it has been observed that the eyes originate on the ventral or sternal side of the body and in the advancement of growth move to the superior side. So according toPackardand others the embryo of Limulus has originally the median eyes on the ventral side. In the same way the cephalic eyes of Dalmanites move, asBarrandehas shown, from the ventral side to their permanent place on the superior side of the head shield. But while in the Cirrhipedia the larval or hypostomic eyes are lost in the adult they remain on the hypostoma of a great number of the adult trilobites and have probably left their mark behind in A pus. No wonder then that we find these maculæ developed as precursors of the hypostomic eyes in a number of the oldest and in other respects blind trilobites as the Olenellidæ and Paradoxidæ to which they may have been of some use as a sort of visual organs, though probably very imperfect.

We find the maculæ of the trilobites present from the oldest Cambrian times and we find also in them a progressive evolution, in some to a high degree, lenses and facets, perfectly identical with those of the eyes on the head shield, converting them into true eyes. It was in the Lower Silurian times, in the Lower grey Orthoceratite limestone of Sweden, that Illænus Esmarki appeared provided with actual prismatic lenses on its maculæ and probably also Lichas. Still higher at the top of the Lower Silurians, in the Leptæna limestone of Dalecarlia the genus Bronteus has appeared with those highly developed hypostomic eyes.

But there are, no doubt, still more facts to adduce for filling up extant lacunæ in the knowledge of these matters. The material now at hand has, however, not been sufficient for doing this. At present I must leave this interesting field of investigation and be contented if I have succeeded in showing where rich harvests await every one who will make a search for them.

All specimens figured belong to the Swedish State Museum,unless otherwise stated.

Acidaspis crenataEmmr.p. 85.Fig. 1 & 2. Vertical sections of the eye 60/1.» 3. The outside of the hypostoma 6/1.» 4. The anterior margin of the hypostoma, showingone of the clubshaped spots in transmittedlight 20/1.» 5. The granulations of the hypostoma in transmittedlight 80/1.» 6. Vertical section of the hypostoma with granules 60/1.Specimens from Djupvik, Eksta.Agnostus glandiformisAngelinp. 35.Fig. 7. Vertical section of a part of a rolled up specimen 8/1.Specimen signed CQ. Andrarum, Scania.Asaphus expansusL.p. 39.Fig. 8. Horizontal section of the eye near the upper margin.a.The surrounding spongious zone,b.The surface of the eye with lenses 60/1.Spec. CG.» 9. Horizontal section from the centrum of the eye 60/1. Spec. CF.» 10. Horizontal section near the inferior margin,athe eye,bmarginal zone,ca portionof the cheek 60/1. Specim. CF.» 11. Vertical section near the inferior margin 60/1,same specimen as in fig. 10.» 12. Eye fractured vertically showing regular prismsin the eye (a) and irregular ones in the marginalzone (b) 30/1.» 13. Part of the hypostoma with the macula 12/1.» 14. The right macula from the inside 12/1.» 15. A vertical section across the hypostoma,aathe maculæ 8/1.» 16. A similar one of the macula at left in thepreceding figure 60/1.» 17. The right macula from the outside in transmittedlight 60/1. Sp. BQ.Asaphus fallaxDalm.?p. 41.Fig. 18. Horizontal section of the eye 60/1. Spec. CD.» 19. Vertical section of the same passing obliquelythrough two rows of prisms 60/1.» 20. Fragment of the same eye with entire prisms 30/1.» 21. The hypostoma seen from the inside 3/1 fromSandvik, Öland.» 22. The left macula 12/1. Spec. BO. From Vestanå, Östergötland.Asaphus ranicepsDalm.p. 40.Fig. 23. The left macula 12/1. Spec. BV.» 24. Horizontal section of right macula 60/1. Spec. BY.» 25. Vertical section of a macula 60/1.» 26. A similar section of a macula (a) in thelongitudinal axis of the hypostoma 12/1.Exterior surface at right.All from Östergötland.Asaphussp. p. 41.Fig. 27. A portion of the eye near the superior margin 30/1.» 28. Horizontal section of the eye 60/1.» 29. Horizontal section near the inferior margin 60/1.» 30. Vertical section near the superior margin 60/1.All figures from specimen CE. From Gotska Sandön.

Acidaspis crenataEmmr.p. 85.

Fig. 1 & 2. Vertical sections of the eye 60/1.» 3. The outside of the hypostoma 6/1.» 4. The anterior margin of the hypostoma, showingone of the clubshaped spots in transmittedlight 20/1.» 5. The granulations of the hypostoma in transmittedlight 80/1.» 6. Vertical section of the hypostoma with granules 60/1.Specimens from Djupvik, Eksta.

Agnostus glandiformisAngelinp. 35.

Fig. 7. Vertical section of a part of a rolled up specimen 8/1.Specimen signed CQ. Andrarum, Scania.

Asaphus expansusL.p. 39.

Fig. 8. Horizontal section of the eye near the upper margin.a.The surrounding spongious zone,b.The surface of the eye with lenses 60/1.Spec. CG.» 9. Horizontal section from the centrum of the eye 60/1. Spec. CF.» 10. Horizontal section near the inferior margin,athe eye,bmarginal zone,ca portionof the cheek 60/1. Specim. CF.» 11. Vertical section near the inferior margin 60/1,same specimen as in fig. 10.» 12. Eye fractured vertically showing regular prismsin the eye (a) and irregular ones in the marginalzone (b) 30/1.» 13. Part of the hypostoma with the macula 12/1.» 14. The right macula from the inside 12/1.» 15. A vertical section across the hypostoma,aathe maculæ 8/1.» 16. A similar one of the macula at left in thepreceding figure 60/1.» 17. The right macula from the outside in transmittedlight 60/1. Sp. BQ.

Asaphus fallaxDalm.?p. 41.

Fig. 18. Horizontal section of the eye 60/1. Spec. CD.» 19. Vertical section of the same passing obliquelythrough two rows of prisms 60/1.» 20. Fragment of the same eye with entire prisms 30/1.» 21. The hypostoma seen from the inside 3/1 fromSandvik, Öland.» 22. The left macula 12/1. Spec. BO. From Vestanå, Östergötland.

Asaphus ranicepsDalm.p. 40.

Fig. 23. The left macula 12/1. Spec. BV.» 24. Horizontal section of right macula 60/1. Spec. BY.» 25. Vertical section of a macula 60/1.» 26. A similar section of a macula (a) in thelongitudinal axis of the hypostoma 12/1.Exterior surface at right.All from Östergötland.

Asaphussp. p. 41.

Fig. 27. A portion of the eye near the superior margin 30/1.» 28. Horizontal section of the eye 60/1.» 29. Horizontal section near the inferior margin 60/1.» 30. Vertical section near the superior margin 60/1.All figures from specimen CE. From Gotska Sandön.

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Asaphus(Isotelus)gigasJ. Hallp. 42.Fig. 1. The right macula surrounded by terrace lines 3/1.» 2. Horizontal section of a macula with a part of the test 60/1.» 3. Vertical section of the same, the surface ofthe outside upwards 60/1. All specimens fromCincinnati, Ohio, Hudson River group. Spec. BU.Bronteus irradiansLindström. p. 43.Fig. 4. The hypostoma 4/1.» 5. The left macula 12/1. Specimen from Gotland, Löjsta.Bronteus laticaudaAng.p. 44.Fig. 6. Horizontal section of the cephalic eye 20/1.» 7. The same highly magnified 250/1.» 8 & 9. Vertical sections of the cephalic eyes 20/1.In fig. 8 with integument, in fig. 9 decorticated.» 10. Hypostoma. Specim. N. 2/1.» 11. The right macula, from the outside. Spec. O. 12/1.» 12 & 13. Left maculæ, casts of the interior surface 12/1.Specimens from Dalecarlia.Bronteus platyactinAng.p. 43.Fig. 14. Vertical section of cephalic eye,athe eye.b, cheek 20/1.» 15. Hypostoma 2/1.» 16. The same from the interior surface. Socketsof the maculæ sunk in the shell 2/1.» 17. The right macula 12/1.» 18. Portion of the eye on the macula in transmitted light 20/1.» 19. The same more magnified 60/1. Specimensfrom the stratumb. Wisby, Norderstrand.Bronteus polyactinAng.p. 8.Fig. 20. Horizontal section of cephalic eye 20/1.» 21. Vertical section of the same 60/1.» 22. Hypostoma 4/1.» 23-25. Samples of right maculæ 20/1.» 26. Horizontal section of macula, the portionbearing the lenses 20/1.» 27. Vertical section of the same 60/1,athelenses. Specimen from Stora Carlsö.» 28. Hypostoma 4/1.» 29. The right macula 12/1.» 30. A portion of the lenses on the macula in reflectedlight 30/1. The lenses are only faintly convex andencased in a white frame. The diameter of the largestlenses 0,06 millim. Specimen from Lansa, Fårö.Bumastus barriensisMurch.p. 46.Fig. 31. Hypostoma 4/1.» 32. The right macula 12/1. Specim. CU from Wisby.Bumastus sulcatusLindst.p. 46.Fig. 33. Horizontal Section of a cephalic eye 20/1.» 34. The same magnified 60/1.» 35. A similar, near the inferior margin and closeto the surface,athe eye,bcheek 60/1.Compare fig. 38.» 36. Vertical section of the cephalic eye near thesuture 20/1.» 37. The same, more magnified 60/1.» 38. A similar, near the inferior margin,aeye,bcheek 60/1.» 39. Cast of the interior surface of a hypostoma 2/1.» 40. The same seen from the side.Originals to figs. 33-38 from Samsugu,Othem, Gotland, figs. 39 & 40 from StoraVede, Follingbo, Gotland.Bumastussp. p. 47.Fig. 41. Horizontal section of cephalic eye, near the inferior margin,aeye,bcheek, 60/1.» 42. Vertical section of the same 60/1.» 43. A similar near the inferior margin,aeye,bcheek, 60/1.» 44. Hypostoma 4/1.» 45. Side view of the same.» 46. The right macula 12/1. Specimens from Korpklint near Wisby.Bumastussp. p. 46.Fig. 47. Hypostoma 4/1.» 48. The right macula 12/1. Specimen from Klints, Othem, Gotland.

Asaphus(Isotelus)gigasJ. Hallp. 42.

Fig. 1. The right macula surrounded by terrace lines 3/1.» 2. Horizontal section of a macula with a part of the test 60/1.» 3. Vertical section of the same, the surface ofthe outside upwards 60/1. All specimens fromCincinnati, Ohio, Hudson River group. Spec. BU.

Bronteus irradiansLindström. p. 43.

Fig. 4. The hypostoma 4/1.» 5. The left macula 12/1. Specimen from Gotland, Löjsta.

Bronteus laticaudaAng.p. 44.

Fig. 6. Horizontal section of the cephalic eye 20/1.» 7. The same highly magnified 250/1.» 8 & 9. Vertical sections of the cephalic eyes 20/1.In fig. 8 with integument, in fig. 9 decorticated.» 10. Hypostoma. Specim. N. 2/1.» 11. The right macula, from the outside. Spec. O. 12/1.» 12 & 13. Left maculæ, casts of the interior surface 12/1.Specimens from Dalecarlia.

Bronteus platyactinAng.p. 43.

Fig. 14. Vertical section of cephalic eye,athe eye.b, cheek 20/1.» 15. Hypostoma 2/1.» 16. The same from the interior surface. Socketsof the maculæ sunk in the shell 2/1.» 17. The right macula 12/1.» 18. Portion of the eye on the macula in transmitted light 20/1.» 19. The same more magnified 60/1. Specimensfrom the stratumb. Wisby, Norderstrand.

Bronteus polyactinAng.p. 8.

Fig. 20. Horizontal section of cephalic eye 20/1.» 21. Vertical section of the same 60/1.» 22. Hypostoma 4/1.» 23-25. Samples of right maculæ 20/1.» 26. Horizontal section of macula, the portionbearing the lenses 20/1.» 27. Vertical section of the same 60/1,athelenses. Specimen from Stora Carlsö.» 28. Hypostoma 4/1.» 29. The right macula 12/1.» 30. A portion of the lenses on the macula in reflectedlight 30/1. The lenses are only faintly convex andencased in a white frame. The diameter of the largestlenses 0,06 millim. Specimen from Lansa, Fårö.

Bumastus barriensisMurch.p. 46.

Fig. 31. Hypostoma 4/1.» 32. The right macula 12/1. Specim. CU from Wisby.

Bumastus sulcatusLindst.p. 46.

Fig. 33. Horizontal Section of a cephalic eye 20/1.» 34. The same magnified 60/1.» 35. A similar, near the inferior margin and closeto the surface,athe eye,bcheek 60/1.Compare fig. 38.» 36. Vertical section of the cephalic eye near thesuture 20/1.» 37. The same, more magnified 60/1.» 38. A similar, near the inferior margin,aeye,bcheek 60/1.» 39. Cast of the interior surface of a hypostoma 2/1.» 40. The same seen from the side.Originals to figs. 33-38 from Samsugu,Othem, Gotland, figs. 39 & 40 from StoraVede, Follingbo, Gotland.

Bumastussp. p. 47.

Fig. 41. Horizontal section of cephalic eye, near the inferior margin,aeye,bcheek, 60/1.» 42. Vertical section of the same 60/1.» 43. A similar near the inferior margin,aeye,bcheek, 60/1.» 44. Hypostoma 4/1.» 45. Side view of the same.» 46. The right macula 12/1. Specimens from Korpklint near Wisby.

Bumastussp. p. 46.

Fig. 47. Hypostoma 4/1.» 48. The right macula 12/1. Specimen from Klints, Othem, Gotland.

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Bumastusn. sp. p. 47.Fig. 1. Hypostoma 4/1. Specimen AY. From Wisby, Norderstrand.» 2. Its left macula 12/1.Bumastusn. sp. (different from the preceding) p. 47.Fig. 3. Hypostoma 4/1. Spec. AT from Wisby, Norderstrand.» 4. Vertical section of the maculaain its longest axis.» 5. A portion of the cephalic eye, the facets very low.Calymmene intermediaLindstr.p. 48.Fig. 6. Hypostoma 4/1. Spec. BG from Gotland.» 7. The left macula 12/1.Calymmene tuberculataBrünnichp. 48Fig. 8. The inside of the hypostoma 3/1. Spec. BA from Gotland.» 9. Horizontal section near the surface, seen from theinside 20/1.Chasmops macroura(?) p. 49.Fig. 10. The left macula, cast of the inside 12/1. Rostock.Chirurus glaberAng.p. 50.Fig. 11. A portion of the surface of the cephalic eye 12/1.From Dalecarlia.Chirurus ornatusDalm.p. 50.Fig. 12. Horizontal section of a Cephalic eye near the surface 60/1.» 13. Vertical section of the same, 20/1, at the inferiormargin. A line added to show the supposed size of thefragmentary lenses.» 14. The left macula of the hypostoma 12/1.From Östergötland, Westanå.Chirurus spinulosusNieczkowskip. 51.Fig. 15. The right macula of the hypostoma from theinside 12/1. Belongs to Dr.G. Holm, fromEsthonia, Kuckers C^2.Chirurussp. p. 51.Fig. 16. The left hypostomic macula 12/1. Öland, Brunsby kanal.Cyrtometopus clavifronsDalm.p. 49.Fig. 17 & 18. Horizontal sections of cephalic eyes 60/1.» 19. Vertical sections of cephalic eyes and a partof the cheek 20/1.» 20. A highly magnified portion of the eye in vertical section60/1.» 21. Hypostoma, 3/1 the anterior wings completed after anotherspecimen. From Östergötland.Cyphaspis elegantulaAngelinp. 52.Fig. 22 & 23. Horizontal and Vertical sections of cephalic eyes 60/1.Diameter of facets 0,03 mm.» 24 & 25. Hypostomas from the front side andin side view 8/1. Upper Silurian, Gotland.Ctenopyge pectenSalterp. 29.Fig. 26. The head shield, the free cheeks added from otherspecimens 4/1.» 27. A free cheek, viewed without foreshortening, as in itsnatural position 4/1. Alumschists of Andrarum, ScaniaCtenopygen. sp. (Linnarsson, Geol. För. Förhandl.1880, pl. VI, figs. 14, 15) p. 29.Fig. 28. A free cheek 6/1.» 29. Surface of the eyes, near the suture 30/1.» 30. The same next the free cheek 30/1. Andrarum.Sphærophthalmus alatusAng.p. 29.Fig. 31. The head shield; the free cheeks have been added 8/1.» 32. A free cheek without foreshortening 8/1.» 33 & 34. Horizontal and Vertical sections of cephalic eyes100/1. The cornea nearly 0,01 mm. in thickness. Andrarum.Peltura scarabæoidesWahlenb.p. 29.Fig. 35. The head shield; the free cheeks added 2/1.» 36. A free cheek, not foreshortened 4/1.» 37. A cast of the inside of the cephalic eye 20/1.» 38. A portion of the cephalic eye from the inside 60/1.» 39. A portion of the same in transmitted light 100/1.» 40. Vertical section of an eye 100/1. The rock on the inside.» 41. A similar one passing through two rows of lenses, the onebehind the other. The rock on the outside 100/1.» 42. A hypostoma 8/1. Andrarum.Dalmanites imbricatulusAngelinp. 53.Fig. 43. The cephalic eye near the inferior margin 20/1.» 44. Vertical section of the same and a portion of the freecheek 60/1. The lenses are almost contiguous and theinterspace between them is formed by the common test ofthe body, which is perforated by longitudinal tubes.Dalmanites obtususLindstr.p. 53.Fig. 45. Horizontal section of cephalic eye near the inferiormargin 30/1.» 46. Vertical section, the interspaces being in this wider thanin the preceding 60/1. From Gotland.Dalmanites vulgarisSalterP. 54.Fig. 47. Part of the cephalic eye near the inferior margin 20/1.» 48. Horizontal section near the surface at the inferiormargin 30/1.» 49. A tangential section, so that the inferior part is belowthe lenses where the peculiar radiating structure is seenin horizontal section 30/1.» 50. Vertical section of a small Specimen, fibrous appendicesbelow the lenses 30/1.» 51. Hypostoma 2/1.» 52. The right macula 12/1. English specimens from Dudley,Shropshire.Dysplanus centrotusDalmanp. 55.Fig. 53 & 54. Horizontal and vertical sections of an eye 60/1.» 55. Hypostoma 3/1.» 56. The anterior wing of the hypostoma seen in foreshortening6/1. Specimen AX from Westanå, Östergötland.Dysplanus ladogensisHolmp. 55.Fig. 57. Hypostoma 3/1.» 58. The same, seen from the side 3/1. Specimen AI fromÖstergötland.

Bumastusn. sp. p. 47.

Fig. 1. Hypostoma 4/1. Specimen AY. From Wisby, Norderstrand.» 2. Its left macula 12/1.

Bumastusn. sp. (different from the preceding) p. 47.

Fig. 3. Hypostoma 4/1. Spec. AT from Wisby, Norderstrand.» 4. Vertical section of the maculaain its longest axis.» 5. A portion of the cephalic eye, the facets very low.

Calymmene intermediaLindstr.p. 48.

Fig. 6. Hypostoma 4/1. Spec. BG from Gotland.» 7. The left macula 12/1.

Calymmene tuberculataBrünnichp. 48

Fig. 8. The inside of the hypostoma 3/1. Spec. BA from Gotland.» 9. Horizontal section near the surface, seen from theinside 20/1.

Chasmops macroura(?) p. 49.

Fig. 10. The left macula, cast of the inside 12/1. Rostock.

Chirurus glaberAng.p. 50.

Fig. 11. A portion of the surface of the cephalic eye 12/1.From Dalecarlia.

Chirurus ornatusDalm.p. 50.

Fig. 12. Horizontal section of a Cephalic eye near the surface 60/1.» 13. Vertical section of the same, 20/1, at the inferiormargin. A line added to show the supposed size of thefragmentary lenses.» 14. The left macula of the hypostoma 12/1.From Östergötland, Westanå.

Chirurus spinulosusNieczkowskip. 51.

Fig. 15. The right macula of the hypostoma from theinside 12/1. Belongs to Dr.G. Holm, fromEsthonia, Kuckers C^2.

Chirurussp. p. 51.

Fig. 16. The left hypostomic macula 12/1. Öland, Brunsby kanal.

Cyrtometopus clavifronsDalm.p. 49.

Fig. 17 & 18. Horizontal sections of cephalic eyes 60/1.» 19. Vertical sections of cephalic eyes and a partof the cheek 20/1.» 20. A highly magnified portion of the eye in vertical section60/1.» 21. Hypostoma, 3/1 the anterior wings completed after anotherspecimen. From Östergötland.

Cyphaspis elegantulaAngelinp. 52.

Fig. 22 & 23. Horizontal and Vertical sections of cephalic eyes 60/1.Diameter of facets 0,03 mm.» 24 & 25. Hypostomas from the front side andin side view 8/1. Upper Silurian, Gotland.

Ctenopyge pectenSalterp. 29.

Fig. 26. The head shield, the free cheeks added from otherspecimens 4/1.» 27. A free cheek, viewed without foreshortening, as in itsnatural position 4/1. Alumschists of Andrarum, Scania

Ctenopygen. sp. (Linnarsson, Geol. För. Förhandl.1880, pl. VI, figs. 14, 15) p. 29.

Fig. 28. A free cheek 6/1.» 29. Surface of the eyes, near the suture 30/1.» 30. The same next the free cheek 30/1. Andrarum.

Sphærophthalmus alatusAng.p. 29.

Fig. 31. The head shield; the free cheeks have been added 8/1.» 32. A free cheek without foreshortening 8/1.» 33 & 34. Horizontal and Vertical sections of cephalic eyes100/1. The cornea nearly 0,01 mm. in thickness. Andrarum.

Peltura scarabæoidesWahlenb.p. 29.

Fig. 35. The head shield; the free cheeks added 2/1.» 36. A free cheek, not foreshortened 4/1.» 37. A cast of the inside of the cephalic eye 20/1.» 38. A portion of the cephalic eye from the inside 60/1.» 39. A portion of the same in transmitted light 100/1.» 40. Vertical section of an eye 100/1. The rock on the inside.» 41. A similar one passing through two rows of lenses, the onebehind the other. The rock on the outside 100/1.» 42. A hypostoma 8/1. Andrarum.

Dalmanites imbricatulusAngelinp. 53.

Fig. 43. The cephalic eye near the inferior margin 20/1.» 44. Vertical section of the same and a portion of the freecheek 60/1. The lenses are almost contiguous and theinterspace between them is formed by the common test ofthe body, which is perforated by longitudinal tubes.

Dalmanites obtususLindstr.p. 53.

Fig. 45. Horizontal section of cephalic eye near the inferiormargin 30/1.» 46. Vertical section, the interspaces being in this wider thanin the preceding 60/1. From Gotland.

Dalmanites vulgarisSalterP. 54.

Fig. 47. Part of the cephalic eye near the inferior margin 20/1.» 48. Horizontal section near the surface at the inferiormargin 30/1.» 49. A tangential section, so that the inferior part is belowthe lenses where the peculiar radiating structure is seenin horizontal section 30/1.» 50. Vertical section of a small Specimen, fibrous appendicesbelow the lenses 30/1.» 51. Hypostoma 2/1.» 52. The right macula 12/1. English specimens from Dudley,Shropshire.

Dysplanus centrotusDalmanp. 55.

Fig. 53 & 54. Horizontal and vertical sections of an eye 60/1.» 55. Hypostoma 3/1.» 56. The anterior wing of the hypostoma seen in foreshortening6/1. Specimen AX from Westanå, Östergötland.

Dysplanus ladogensisHolmp. 55.

Fig. 57. Hypostoma 3/1.» 58. The same, seen from the side 3/1. Specimen AI fromÖstergötland.

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Chirurus speciosusHis.Fig. 1. The right macula, cast of the interior side 12/1.Specimen from Grumpevik, Vamlingbo, Gotland.Cybele bellatulaDalm.p. 52.Fig. 2. The hypostoma 3/1. Vestanå, Östergötland.Dalmanites scleropsDalmanp. 54.Fig. 3. The hypostoma 2/1.Encrinurus punctatusWahlenb.p. 56.Fig. 4. Horizontal Section of an eye 69/1. Diameterof the prisms 0,04 mill., their length 0,07 millim.» 5. The hypostoma 3/1.» 6. The left macula. Reflected light 12/1.» 7. Part of the left macula in transmitted light 20/1.The black streaks are sectioned pores. Spec. BH.» 8. Vertical section of the right macula, a sagittalsection 20/1. BH.» 9. A similar oneaalong the shorter diameter 20/1.Specimens 5, 6 from Vikers Boge, 4, 9 Medebys, Hall,7, 8 Svarfvarehuk. All from Gotland.Encrinurus lævisAng.p. 56.Fig. 10. Horizontal section of the cephalic eye.» 11. Vertical section of the same. Both 60/1.Prisms 0,05 in breadth, 0,06 in length. Specimenfrom Wisby.» 12-17. Longitudinal profiles and anterior margins of thehypostomas of Encr. punctatus (figs. 12, 13),E. obtusus (figs. 14, 15) and E. lævis (figs. 16, 17).Harpes vittatusBarr.p. 31.Fig. 18. The two ocelli at right 8/1.» 19. Sagittal Section of two ocelli 60/1. Specimen from LochkowE, e^2, Bohemia.Homalonotus KnightiKönigp. 57.Fig. 20. The hypostoma 3/2.» 21. The right macula, from the interior side in transmittedlight 20/1. From Scania.Illænus chironHolmp. 58.Fig. 22. Horizontal Section of an eye.» 23. Vertical section of the same, both 60/1. Spec. AG.» 24. Hypostoma 2/1. Spec. AF.» 25. The right macula seen in reflected light. The lensestranslucent 12/1. Ex. AA. All from Persnäs, Öland.Illænus EsmarkiSchloth.p. 511.Fig. 26 & 27. Horizontal sections of the eyes in two differentspecimens 60/1.» 28. Vertical section of an eye 60/1.» 29. Likewise,athe eye,ba portion of thecheek 12/1.» 30. Hypostoma 2/1.» 31. The inside of the right macula 12/1. Spec. AR.» 32. The outside of the left macula in transmitted light 60/1.» 33. Sagittal section of the macula (a) 60/1.All specimens from Vestanå, Östergötland, exceptingfig. 30 belonging to Dr.Holm, fromNorway, Bjerkaas.Illænus gigasHolmP. 59.Fig. 34 & 35. Surface views of the prisms of the eyes, fig. 34 atthe superior margin and 35 at the inferior one 12/1.» 36. Cast of the inside of a hypostoma 1/1. Same original asHolm'sfig. 6 Tafl. I, Illænider.» 37. Cast of the inside of the left macula. Same specimen 12/1.Illænus RoemeriVollborth(Ill. vivaxHolm)p. 59.Fig. 38. Hypostoma 1/1. Spec. AO.» 39. The right macula; the lenses translucent in reflected light12/1. Spec. AO.» 40, 41. Both maculæ of another, larger specimen 12/1. Sp. AP.From the Leptæna limestone of Dalecarlia.Illænus sphæricusHolmp. 59.Fig. 42. The prisms of cephalic eye 12/1. }» 43. The right maculæ, transparent in }transmitted light 12/1. Sollerön, } Spec. AH.Dalecarlia. }Lichassp. p. 60.Fig. 44. The hypostoma 1/1.» 45. Cast of the inside of the right macula 12/1. Spec. T.» 46. Cast in wax of the preceding 12/1.» 47. Cast of the inside of the hypostoma. The central portionwith the maculæ 3/1. Spec. V.» 48. Inside of the right macula 12/1. Spec. Y.From a quarry north of the gallows near Wisby.Lichas latifronsAng.p. 60.Fig. 49. The right macula, transmitted light, seen from the inside60/1. From Kyrkberget, Wisby.Platymetopus planifronsAng.p. 67.Fig. 50. The hypostoma 1/1.» 51. The left macula. Transparent lenses? 12/1.From the Leptæna limestone of Dalecarlia.Trochurus pusillusAng.p. 70.Fig. 52. The hypostoma 5/1.» 53. The right macula in transmitted light. Seen from the inside60/1. From Fröjel.Trochurus?p. 70.Fig. 54. The hypostoma 3/1.» 55. The right macula from the inside 12/1. From När Gotland.

Chirurus speciosusHis.

Fig. 1. The right macula, cast of the interior side 12/1.Specimen from Grumpevik, Vamlingbo, Gotland.

Cybele bellatulaDalm.p. 52.

Fig. 2. The hypostoma 3/1. Vestanå, Östergötland.

Dalmanites scleropsDalmanp. 54.

Fig. 3. The hypostoma 2/1.

Encrinurus punctatusWahlenb.p. 56.

Fig. 4. Horizontal Section of an eye 69/1. Diameterof the prisms 0,04 mill., their length 0,07 millim.» 5. The hypostoma 3/1.» 6. The left macula. Reflected light 12/1.» 7. Part of the left macula in transmitted light 20/1.The black streaks are sectioned pores. Spec. BH.» 8. Vertical section of the right macula, a sagittalsection 20/1. BH.» 9. A similar oneaalong the shorter diameter 20/1.Specimens 5, 6 from Vikers Boge, 4, 9 Medebys, Hall,7, 8 Svarfvarehuk. All from Gotland.

Encrinurus lævisAng.p. 56.

Fig. 10. Horizontal section of the cephalic eye.» 11. Vertical section of the same. Both 60/1.Prisms 0,05 in breadth, 0,06 in length. Specimenfrom Wisby.» 12-17. Longitudinal profiles and anterior margins of thehypostomas of Encr. punctatus (figs. 12, 13),E. obtusus (figs. 14, 15) and E. lævis (figs. 16, 17).

Harpes vittatusBarr.p. 31.

Fig. 18. The two ocelli at right 8/1.» 19. Sagittal Section of two ocelli 60/1. Specimen from LochkowE, e^2, Bohemia.

Homalonotus KnightiKönigp. 57.

Fig. 20. The hypostoma 3/2.» 21. The right macula, from the interior side in transmittedlight 20/1. From Scania.

Illænus chironHolmp. 58.

Fig. 22. Horizontal Section of an eye.» 23. Vertical section of the same, both 60/1. Spec. AG.» 24. Hypostoma 2/1. Spec. AF.» 25. The right macula seen in reflected light. The lensestranslucent 12/1. Ex. AA. All from Persnäs, Öland.

Illænus EsmarkiSchloth.p. 511.

Fig. 26 & 27. Horizontal sections of the eyes in two differentspecimens 60/1.» 28. Vertical section of an eye 60/1.» 29. Likewise,athe eye,ba portion of thecheek 12/1.» 30. Hypostoma 2/1.» 31. The inside of the right macula 12/1. Spec. AR.» 32. The outside of the left macula in transmitted light 60/1.» 33. Sagittal section of the macula (a) 60/1.All specimens from Vestanå, Östergötland, exceptingfig. 30 belonging to Dr.Holm, fromNorway, Bjerkaas.

Illænus gigasHolmP. 59.

Fig. 34 & 35. Surface views of the prisms of the eyes, fig. 34 atthe superior margin and 35 at the inferior one 12/1.» 36. Cast of the inside of a hypostoma 1/1. Same original asHolm'sfig. 6 Tafl. I, Illænider.» 37. Cast of the inside of the left macula. Same specimen 12/1.

Illænus RoemeriVollborth(Ill. vivaxHolm)p. 59.Fig. 38. Hypostoma 1/1. Spec. AO.» 39. The right macula; the lenses translucent in reflected light12/1. Spec. AO.» 40, 41. Both maculæ of another, larger specimen 12/1. Sp. AP.From the Leptæna limestone of Dalecarlia.

Illænus sphæricusHolmp. 59.

Fig. 42. The prisms of cephalic eye 12/1. }» 43. The right maculæ, transparent in }transmitted light 12/1. Sollerön, } Spec. AH.Dalecarlia. }

Lichassp. p. 60.

Fig. 44. The hypostoma 1/1.» 45. Cast of the inside of the right macula 12/1. Spec. T.» 46. Cast in wax of the preceding 12/1.» 47. Cast of the inside of the hypostoma. The central portionwith the maculæ 3/1. Spec. V.» 48. Inside of the right macula 12/1. Spec. Y.From a quarry north of the gallows near Wisby.

Lichas latifronsAng.p. 60.

Fig. 49. The right macula, transmitted light, seen from the inside60/1. From Kyrkberget, Wisby.

Platymetopus planifronsAng.p. 67.

Fig. 50. The hypostoma 1/1.» 51. The left macula. Transparent lenses? 12/1.From the Leptæna limestone of Dalecarlia.

Trochurus pusillusAng.p. 70.

Fig. 52. The hypostoma 5/1.» 53. The right macula in transmitted light. Seen from the inside60/1. From Fröjel.

Trochurus?p. 70.

Fig. 54. The hypostoma 3/1.» 55. The right macula from the inside 12/1. From När Gotland.

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