U. bigbyensis Lea, ’41. U. estabrookianus Lea, ’45. U. fassinans Lea, ’68. Pleurobema fassinans rhomboidea Simpson, ’00.
The most frequent form is fassinans rhomboidea (topotypes examined), with rays poorly developed. U. bigbyensis has more distinct rays; U. estabrookianus (topotypes examined) is an old, overgrown form, without rays; U. fassinans is founded upon an individual (type examined, also topotypes), which is exceptionally elongated, without rays.
3. Fusconaia barnesiana tumescens (Lea) 1845.
U. tumescens Lea, ’45. U. crudus Lea, ’71. U. radiosus Lea, ’71.
U. tumescens is 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 like tumescens; U. crudus (topotypes examined) lacks rays, and has much eroded beaks, but stands close to radiosus.
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 only one (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.