U. tumescensLea, ’45.U. crudusLea, ’71.U. radiosusLea, ’71.
U. tumescensis the most typical form, greatly swollen, with more or less developed rays;U. radiosus(type and topotypes examined) is less swollen, but for the rest liketumescens;U. crudus(topotypes examined) lacks rays, and has much eroded beaks, but stands close toradiosus.
The mutual relations of these forms may be understood by the help of the following key. Only the three largest divisions are to be regarded as varieties, in the other forms the characters are merely individual, although specimens representing onlyone (or a few) of these “forms” often prevail at a given locality.
a1. Flat, compressed, dia. of shell less than 40 per cent of the length (var.bigbyensis).b1. No rays, or rays obscure, color of epidermis brown, dark.c1. Rhomboid in shape.d1. Large.Estabrookianus.d2. Smaller.Fassinans rhomboidea.c2. More ovate, tapering behind.Fassinans.b2. Rays distinct, well developed over most of the disk. Ground color of epidermis lighter.Bigbyensis.a2. Moderately convex, dia. 40–49 per cent of length.Barnesiana typica.b1. Beaks not elevated, shape trapezoidal, rhomboid, or subovate.c1. Dia. about 41 or 42 per cent; size small.d1. Shape somewhat elongate (trapezoidal); rays obscure.Barnesianus.d2. Shape shorter (rhomboidal).e1. Rays obscure.Lenticularis.e2. Rays present, color of epidermis lighter.f1. Rays few.Meredithi.f2. Rays numerous.Pudicus.c2. Dia. about 45 per cent; larger. Shape subovate. Rays obscure.Tellicoensis.b2. Beaks more elevated, shape subtriangular. Dia. 46 per cent, with rather distinct rays.Lyoni.a3. Much swollen, dia. over 50 per cent. Beaks elevated. (var.Tumescens).b1. Without rays. Dia. 51 per cent. Beaks much eroded.Crudus.b2. With rays. Dia. about 56 per cent or more.c1. Dia. about 56 per cent.Radiosus.c2. Dia. about 64 per cent.Tumescens.
a1. Flat, compressed, dia. of shell less than 40 per cent of the length (var.bigbyensis).b1. No rays, or rays obscure, color of epidermis brown, dark.c1. Rhomboid in shape.d1. Large.Estabrookianus.d2. Smaller.Fassinans rhomboidea.c2. More ovate, tapering behind.Fassinans.b2. Rays distinct, well developed over most of the disk. Ground color of epidermis lighter.Bigbyensis.a2. Moderately convex, dia. 40–49 per cent of length.Barnesiana typica.b1. Beaks not elevated, shape trapezoidal, rhomboid, or subovate.c1. Dia. about 41 or 42 per cent; size small.d1. Shape somewhat elongate (trapezoidal); rays obscure.Barnesianus.d2. Shape shorter (rhomboidal).e1. Rays obscure.Lenticularis.e2. Rays present, color of epidermis lighter.f1. Rays few.Meredithi.f2. Rays numerous.Pudicus.c2. Dia. about 45 per cent; larger. Shape subovate. Rays obscure.Tellicoensis.b2. Beaks more elevated, shape subtriangular. Dia. 46 per cent, with rather distinct rays.Lyoni.a3. Much swollen, dia. over 50 per cent. Beaks elevated. (var.Tumescens).b1. Without rays. Dia. 51 per cent. Beaks much eroded.Crudus.b2. With rays. Dia. about 56 per cent or more.c1. Dia. about 56 per cent.Radiosus.c2. Dia. about 64 per cent.Tumescens.
a1. Flat, compressed, dia. of shell less than 40 per cent of the length (var.bigbyensis).
a1. Flat, compressed, dia. of shell less than 40 per cent of the length (var.bigbyensis).
b1. No rays, or rays obscure, color of epidermis brown, dark.
b1. No rays, or rays obscure, color of epidermis brown, dark.
c1. Rhomboid in shape.
c1. Rhomboid in shape.
d1. Large.Estabrookianus.
d1. Large.Estabrookianus.
d2. Smaller.Fassinans rhomboidea.
d2. Smaller.Fassinans rhomboidea.
c2. More ovate, tapering behind.Fassinans.
c2. More ovate, tapering behind.Fassinans.
b2. Rays distinct, well developed over most of the disk. Ground color of epidermis lighter.Bigbyensis.
b2. Rays distinct, well developed over most of the disk. Ground color of epidermis lighter.Bigbyensis.
a2. Moderately convex, dia. 40–49 per cent of length.Barnesiana typica.
a2. Moderately convex, dia. 40–49 per cent of length.Barnesiana typica.
b1. Beaks not elevated, shape trapezoidal, rhomboid, or subovate.
b1. Beaks not elevated, shape trapezoidal, rhomboid, or subovate.
c1. Dia. about 41 or 42 per cent; size small.
c1. Dia. about 41 or 42 per cent; size small.
d1. Shape somewhat elongate (trapezoidal); rays obscure.Barnesianus.
d1. Shape somewhat elongate (trapezoidal); rays obscure.Barnesianus.
d2. Shape shorter (rhomboidal).
d2. Shape shorter (rhomboidal).
e1. Rays obscure.Lenticularis.
e1. Rays obscure.Lenticularis.
e2. Rays present, color of epidermis lighter.
e2. Rays present, color of epidermis lighter.
f1. Rays few.Meredithi.
f1. Rays few.Meredithi.
f2. Rays numerous.Pudicus.
f2. Rays numerous.Pudicus.
c2. Dia. about 45 per cent; larger. Shape subovate. Rays obscure.Tellicoensis.
c2. Dia. about 45 per cent; larger. Shape subovate. Rays obscure.Tellicoensis.
b2. Beaks more elevated, shape subtriangular. Dia. 46 per cent, with rather distinct rays.Lyoni.
b2. Beaks more elevated, shape subtriangular. Dia. 46 per cent, with rather distinct rays.Lyoni.
a3. Much swollen, dia. over 50 per cent. Beaks elevated. (var.Tumescens).
a3. Much swollen, dia. over 50 per cent. Beaks elevated. (var.Tumescens).
b1. Without rays. Dia. 51 per cent. Beaks much eroded.Crudus.
b1. Without rays. Dia. 51 per cent. Beaks much eroded.Crudus.
b2. With rays. Dia. about 56 per cent or more.
b2. With rays. Dia. about 56 per cent or more.
c1. Dia. about 56 per cent.Radiosus.
c1. Dia. about 56 per cent.Radiosus.
c2. Dia. about 64 per cent.Tumescens.
c2. Dia. about 64 per cent.Tumescens.
As to the geographical distribution, it should be briefly stated that the swollen forms (a3) inhabit the largest rivers; the compressed forms (a1) are found in the headwaters, and the intermediateforms (a2) belong to the streams of moderate size. Intergrades are frequent.