NEPHRONAIAS. This genus has for its type the _Unio plicatulus_, Küster, a species identified by von Martens as belonging to the Lampsiline shells, as _aztecorum_. Mr. Simpson however believed it to be nearly allied to the _persulcatus_, a markedly Unioid shell. In this the writer follows Mr. Simpson. The genus _Nephronaias_ as constituted by Mr. Simpson embraces two quite distinct groups, divisible as follows. _Nephronaias_ (_s. s._) embraces _plicatulus_, _persulcatus_, _melleus_, _dysoni_, _ortmanni_, _ravistellus_, etc. Ample material of these two latter species show that they are anatomically very closely allied to _Elliptio_. There is no sexual difference of shape, and the gill is gravid in its whole length. _Nephronaias_ differs from _Elliptio_ in its sulcated disc, in its beak sculpturing, etc. Included in _Nephronaias_ by Simpson are, however, shells of a totally different type, such as _medellinus_, _gundlachi_, _sapotalensis_, etc. These latter are sexually dimorphic, smoother, more generally rayed, and the gravid uterus is of Lampsiline type. The position of the dorsal scars within the beak cavities is different, in the examples of the pseudo _Nephronaias_ seen by the writer. _Nephronaias_ (_s. s._) possesses an (accessory?) adductor scar attached to the frontal portion of the cardinal teeth, which is either absent or obsoletely marked in the second assemblage. For this latter group the writer, therefore, proposes to use the generic term of _Actinonaias_ Crosse and Fischer, 1893, type _U. sapotalensis_ Lea. The female of this species has been described by Dr. Ortmann (1912). _Actinonaias_ embraces, besides the type, _medellinus_, _gundlachi_, (accepting Simpson’s interpretation of this latter species), and others. PSORONAIAS, Crosse and Fischer (1893). This group of remarkable shells, embracing _crocodilarum_, _psoricus_, _semigranosus_, etc., was provisionally treated by Simpson as a group of _Elliptio_, but their remarkable sculpturing, and the deep beak cavities of some of their species, led him to observe that it was possible that the group should, after all, be placed in _Quadrula_. I follow von Martens, in giving generic rank as above to the group. The type is _Unio psoricus_. To this genus we are enabled to add a species hitherto undescribed, under the name of PSORONAIAS KUXENSIS, n. sp., PI. VII, figs. 1, 2. Shell small, compressed, rough, brown, biangular. Length 50, height 30, diam. 17 mm.