402 PROCEEDINGS OF THE ACADEMY OF [1899. 



dinal tooth, with a lower, very thin one above it aud slightly more 

 posterior ; 'anterior lateral short, erect, thin and triangular ; pos- 

 terior lateral short and low. Right valve with two subequal, erect, 

 squarish cardinals, one behind the other, separated by a deep 

 V-shaped notch; laterals double, the anterior very short, the pos- 

 terior longer, low. 



Length 15.5, alt. 11, diam. 7.6 mm. 



Tzintzuntzan, Lake Patzcuaro, State of Michoacau, Mexico. 



This is a rather large species, more fragile than those of the 

 non-calyculate group in the L^nited States. Compared with S. 

 subtransverstim Prime, which has the same general outline, this 

 species differs in the broader beaks, Avhich do not show the promi- 

 nent ' ' caps ' ' or prodissoconch of S. suhtransversuvi ; the cardinal 

 teeth are much thinner, more compressed, and the lateral laminse 

 are less elevated. It does not seem closely allied to any of the 

 American species I have been able to compare with it. 



