Occasional Papers of the Museum of Zoology ii 



hinge armature is quite similar to that of E. liebmanni (i. e., 

 the pseudocardinals are more apt to be compressed, quite sim- 

 ple, and more or less oblique — fig. 15, plate III). 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. kuxcnsis 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. 



