[31]Such is the aspect of the organisms in some of the Silurian sandstones near Girvan. I have seen the same bright tint on a set of fossils from the Llandeilo flags of Wales, and from the slates of Desertcreat, Ireland, and have disinterred similarly gilded shells from the vertical greywackè slates of the Pentland Hills and Peeblesshire. Nothing can be more beautiful than the aspect of these fossils when first laid open, but the bright gleam eventually passes away on exposure.
[31]Such is the aspect of the organisms in some of the Silurian sandstones near Girvan. I have seen the same bright tint on a set of fossils from the Llandeilo flags of Wales, and from the slates of Desertcreat, Ireland, and have disinterred similarly gilded shells from the vertical greywackè slates of the Pentland Hills and Peeblesshire. Nothing can be more beautiful than the aspect of these fossils when first laid open, but the bright gleam eventually passes away on exposure.
But there are also some orders that would be best illustrated by a long line of nearly uniform breadth, extending from the first geologic periods to the present day. In other words, they seem to have retained during all time pretty much the same amount of development. I shall confine my notice of the carboniferous Crustacea to the description of a single genusbelonging to a family that seems to have begun during the period of the Lower Silurian, and still flourishes abundantly in existing waters.
Fig. 20.1. Carboniferous cypris, nat. size, and magnified.2. Recent cypris, highly magnified.3. Carboniferous king-crab (Limulus trilobitoides).
Fig. 20.1. Carboniferous cypris, nat. size, and magnified.2. Recent cypris, highly magnified.3. Carboniferous king-crab (Limulus trilobitoides).
1. Carboniferous cypris, nat. size, and magnified.2. Recent cypris, highly magnified.3. Carboniferous king-crab (Limulus trilobitoides).
The genus to which I refer is a well-known fossil in some parts of the Coal-measure series, and has been namedCypris. The shells ofcypridesare very minute, considerably less than the heads of small pins (Fig. 20-1). They can be seen quite well, however, without the use of a magnifying power. In shape they resemble beans, and when seen scattered over a slab of shale, look much liker seeds than the relics of animal life. Yet, under this simple exterior, they concealed a somewhat complex organization. The little bean-shaped shells, which are all that now remains to us of their structure, formed the crust or outer shell in which their viscera were contained, and answered to the massive carapace and segments of the crab. They consisted of two valve-like cases fitting to each other, so as to resemble the united valves of a bivalve shell. From the upper end there were protruded through the opening between the valves a pair of slim jointed antennæ, each furnished at its point with a bundle of minute hair-like cilia (Fig. 20-2). These, when set in rapid motion, served to impel the creature through the water. The legs, four in number, were encrusted with the same hard membrane, and had the same jointed structureas those of our common shrimps and crabs. The foremost pair were pointed like the antennæ with fine hairs, the incessant action of which assisted the animal in swimming. Of the little, confluent, sessile eyes, the delicate branchia or gills, and all the complex internal structure of the nervous, circulating, and other systems, no trace has survived on the stone; but enough of the general external form is left to show us the true affinities of these organisms in the animated world of the present time. By studying the forms and habits of the cyprides that swarm in some of our ponds and marshes, a just conception is obtained of the structure and habitat of the animals that once occupied the minute bean-shaped shells, which lie by millions among the shales of the Carboniferous system. From such a comparison we infer, that just as the cyprides of to-day are fresh-water animals abounding among the green slime of stagnant pools, so, in past ages, they must have preserved with the same organization the same habits. And thus we arrive at the important conclusion that the strata in which the remains of cyprides abound must have been deposited in lakes or rivers. This gives us a key by which to interpret some of the changes of a geological system, and the ancient physical revolutions of large tracts of country.
The shales of the coal-measures sometimes contain the cypris cases in such abundance as to derive therefrom a sort of fissile structure. It should be borne in mind, however, that each animal may during its lifetime have possessed in succession several of these cases. Among the shell-bearing molluscous animals, the little shell which contains the creature in its youngest stages remains ever after as an integral part of the outer calcareous case. As the inhabitant grows, it continues to add band after band to the outer edge of the shell, each of which, whilst preserving the general symmetry and proportions of the whole structure, increases its dimensions in every way. Among the univalves, such, for example, as the turritella, socommon on our shores, the layers of growth succeed each other like the steps of one of those long spiral stairs that our feudal forefathers loved to build from the court-yard to the watch-tower of their castles. Each new layer exceeding in bulk its predecessors, adds a new step to the ascending pile, and thus the ever-widening mouth winds spirally upwards around the central pillar. The bivalves exemplify the same principle. The successive additions are made in a crescent form to the outer edges, and form those prominent concentric ridges so conspicuous on many of our commoner shells, such, for instance, as some of those in the generaAstarteandVenus.
But the architecture of the Crustacea (and, of course, that of the cyprides) is conducted on a very different principle. Their houses admit of no additions or enlargements, and so, when the animals find themselves getting somewhat straitened, they retire to a sheltered spot, and there, separating the walls that hem them in, crawl out like soft lumps of dough. The outer membrane of the moulted animal quickly acquires strength and hardness, and in a day or two the renovated creature is as healthy and vigorous as ever. In this process it is not merely an external shell, like that of a mollusc, which is thrown off, but a veritable skin, so that when the old shell is abandoned it frequently could not be detected on a first glance to be empty, the outer crust of every leg and joint, and sometimes even of thin bristles, remaining just as in the living animal.
It is not unlikely that this process of moulting takes place annually in most of the Crustacea, so that if we suppose a fossil member of the group to have lived six years, it would have left six crusts to be entombed in any deposits that might be forming at the time. Of course there would be many chances against all the six being preserved, but the possibility of at least several of them becoming fossilized should be borne in mind when we speculate on the abundance of such organisms in any geological formation.
I might refer to another very interesting group of crustacean animals known as theLimuli, or king-crabs, of which there were at least three representatives during the times of the English Carboniferous system (Fig. 20-3). They are remarkable chiefly for their large crescent-shaped shield, their long sword-like tail, and their double pair of eyes, of which the outer ones are large, sessile, and compound, like those of the trilobites, while the middle pair are small, simple, and set close together on the forehead, like those of the single-eyed Cyclops in the old mythology. Altogether, with their shields, swords, watchful waking eyes, strong massive armour, and great size (for some of them measure two feet in length), they form a most warlike genus.