Occastonal Papers of the Museum of Zoology II 
hinge armature is quite similar to that of &. liebmanni (i. e., 
the pseudocardinals are more apt to be compressed, quite sim- 
ple, and more or less oblique—fig. 15, plate IIT). However, 
the laterals are always proportionately shorter and more 
curved than in that species, although in this, as in other char- 
acters, the two species somewhat approach each other. As is 
true of all of these shells from the Rio San Juan, the early 
erosion of the beaks and the constant malformation (due to 
tropical floods?) cause many small individuals to have every 
appearance of an adult shell. For this reason, and because the 
specimens in the A. N. S. P. (among them the type) have 
the juvenile pseudocardinals of this species, I am inclined to 
believe that P. kusxensis Frierson (1917) is a depauperate, 
small stream form, closely related to E. plexus crocodilarum 
(Mo.). However, the general appearance is certainly that of 
completely adult shells. 
In all of the specimens of this series of E. plexus distinctus 
the beaks are eroded, but the smaller specimens retain enough 
to lead me to believe that the beak sculpture of this species 
consists of very irregular, but more or less parallel wrinkles, 
disturbed by radial plicae and pustules, with a slight tendency 
to be doubly looped, rounded on the posterior slope and with 
an oblique V on the anterior one, so that the sculpture appears 
to run obliquely postero-ventrad. 
Plate II, fig. 13, shows a very peculiar, compressed and 
slightly sinuate shell, which looks quite like a different species. 
The nacre and the general shape of the inside of the shell, as 
well as the lateral teeth, are quite typical of distinctus, but the 
pseudocardinals are of the juvenile type; that is, they are com- 
pressed, almost equal and oblique (figs. 3 and 15 are similar). 
The beaks are eroded, but the remainder of the shell shows 
no sign of ornamentation, and is almost black, smooth and 
shiny, with evident, fine, radiating striations. Other specimens 
approach it in various characters, but it combines so many 
peculiarities that, if found in lage numbers, it would seem to 
require at least distinct racial recognition. 
