Occasional Papers of the Museum of Zoology 31 



the projections formed by the ventral ends of the two swell- 

 ings. Two older shells have a slight arcuate tendency at the 

 prolonged posterior point, which gives them a peculiar, beaked 

 appearance (fig. 54). In both of these last, the beaks are 

 eroded to such a degree that the pseudocardinals show dorsally 

 as a sinuation of the hinge-hne. 



This swollen shell is more markedly Lampsilis-like in gen- 

 eral appearance than are any of the other southern Mexican 

 naiades ; in fact, if without definite locality, it would undoubt- 

 edly be taken for a shell from the central United States. The 

 marsupial ( Y) swellings even give it a certain resemblance to 

 the genus Truncilla. All of the specimens are heavily eroded, 

 as if the shell-substance was softer than usual, which may be 

 the reason for the ligamental invasion of the hinge plate. 

 Among the Rio San Juan uniones, Lampsilis mthveni is 

 nearest Lampsilis rovirosai sanjuanensis in shape, although 

 much smaller and heavier, while its color and rays give it a 

 superficial resemblance to A. sapoialcnsis. The radiate poste- 

 rior swellings and the general inflation, especially of the 

 females(?j are very distinctive characters in a Mexican form. 



Me-^surements 



