IO 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 
future confusion. ; 
Elliptio (Sphenonaias) plexus, subspecies distinctus (C. and 
) (803)). Plate Inthe. 2 - plates splate UNI ite. i 5saiei— 
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. testudineus 
Morelet. Some of the older specimens have the biangular 
posterior margin and the double ridge of U. morini Mo. It 
seems probable that Lea was quite right when he combined 
E. plexus with E. crocodilarum Mo., as some specimens of this 
intermediate form are indistinguishable from the latter shell. 
E. plexus crocodilarum (Mo.) may still be retained, as a sub- 
species, for the usually larger and more cylindrical, southern 
form. From the variation in E£. plexus distinctus, it seems 
probable that both U. morimi and U. testudineus are synonyms 
of E. plexus crocodilarum, or the second may be a subspecies. 
E. 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 E. psoricus (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 
