_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. a_{1}. Flat, compressed, dia. of shell less than 40 per cent of the length (var. _bigbyensis_). b_{1}. No rays, or rays obscure, color of epidermis brown, dark. c_{1}. Rhomboid in shape. d_{1}. Large. _Estabrookianus._ d_{2}. Smaller. _Fassinans rhomboidea._ c_{2}. More ovate, tapering behind. _Fassinans._ b_{2}. Rays distinct, well developed over most of the disk. Ground color of epidermis lighter. _Bigbyensis._ a_{2}. Moderately convex, dia. 40‒49 per cent of length. _Barnesiana typica._ b_{1}. Beaks not elevated, shape trapezoidal, rhomboid, or subovate. c_{1}. Dia. about 41 or 42 per cent; size small. d_{1}. Shape somewhat elongate (trapezoidal); rays obscure. _Barnesianus._ d_{2}. Shape shorter (rhomboidal). e_{1}. Rays obscure. _Lenticularis._ e_{2}. Rays present, color of epidermis lighter. f_{1}. Rays few. _Meredithi._ f_{2}. Rays numerous. _Pudicus._ c_{2}. Dia. about 45 per cent; larger. Shape subovate. Rays obscure. _Tellicoensis._ b_{2}. Beaks more elevated, shape subtriangular. Dia. 46 per cent, with rather distinct rays. _Lyoni._ a_{3}. Much swollen, dia. over 50 per cent. Beaks elevated. (var. _Tumescens_). b_{1}. Without rays. Dia. 51 per cent. Beaks much eroded. _Crudus._ b_{2}. With rays. Dia. about 56 per cent or more. c_{1}. Dia. about 56 per cent. _Radiosus._ c_{2}. Dia. about 64 per cent. _Tumescens._