Occasional Papers of the Museum of Zoology Bi 
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-line. 
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(?) 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 ruthveni is 
nearest Lampsilis rovirosai sanjuanensis in shape, although 
much smaller and heavier, while its color and rays give it a 
superficial resemblance to A. sapotalensis. ‘The radiate poste- 
rior swellings and the general inflation, especially of the 
females(?) are very distinctive characters in a Mexican form. 
MEASUREMENTS 
~ 
wm a 
BS cae 
ws om 
S u 
ort Os 
yy Vv <= @) 
6 & oe 
Op & 
“— o S) 
O en Sg a) 
an (an) 
66(35)  48(25) 
Or U1 U1 On a 
© AnwW {Length in mm. 
On 
Fig. 52 64(35)  47(26) (male?) 
66(37) 48(27) 
Fig. 51 65 (38) 58(34) (type, female?) 
Fig. 53 60 
72 (43) 52(31) (much higher; female?) 
Fig. 54 73 62(45) 46(33.5) 
73.5 62(45.5) 46(34) 
Means (7 
specimens ) 61.5 65 49 
Extremes 53-73.5 62-72 46-58 
eee Ee eee ea eae 
