lo University of Michigan 



here. Sphenonaias should not go into the synonomy of Psoro- 

 naias, nor vice versa, until the anatomy of a typical example 

 of either section has been studied ; the group whose anatomy 

 is unknown should be placed, at least temporarily, in the 

 synonymy of the one studied, so as to reduce the chances of 

 luture confusion. , 



Elliptio (Sphenonaias) plexus, subspecies distinctus (C. and 

 F.) (1893). Plate I, fig. 3; plate II; plate III, fig. 15, 11.— 

 Thirty-nine specimens, including odd valves, from the Rio 

 San Juan (H, vii, c). This is a very variable form; nothing 

 about it seems constant. The sculpture varies from almost 

 perfectly smooth to plicate (typical forms), and finally pus- 

 tulate. The shape varies from quite close to that of the next 

 species to a quadrate form which resembles U. testiidineus 

 Morelet. Some of the older specimens have the biangular 

 posterior margin and the double ridge oi U. morini Mo. It 

 seems probable that Lea was quite right when he combined 

 B. plexus with E. crocodilanim Mo., as some specimens of this 

 intermediate form are indistinguishable from the latter shell. 

 B. plexus crocodilaruni (Mo.) may still be retained, as a sub- 

 species, for the usually larger and more cylindrical, southern 

 form. From the variation in B. plexus distinctus, it seems 

 probable that both U. morini and U. testiidineus are synonyms 

 of B. plexus erocodilarum, or the second may be a subspecies. 

 B. semigranosus (von dem Busch), from the Panuco River 

 system, is a considerably more compressed form, and may be 

 a separate species, although a series in the A. N. S. P., from 

 the Tecomate River, are more or less intermediate between it 

 and the present form. It is, at least, a very distinct northern 

 subspecies. Unio corium Reeve appears to me to be a distinct 

 species, more closely related to B. psoriciis (Mo.) than to the 

 present group. 



Besides this variation in the larger specimens, this subspecies 

 (distinctus) varies a great deal with age. The younger shells 

 are much more compressed in shape and have a whitish nacre, 

 although they quite early assume the coppery tint. Their 



