Occasional Papers of the Museum of Zoology 23 



of the points mentioned in the original comparison might be 

 appHed to the latter, but I cannot believe that Simpson (1900) 

 would ever confuse a species, even a male specimen, of this 

 group with A. cxplicata. 



A. zvalkcri is smaller than typical disca and, in proportion 

 to size, is also heavier than either disca or finibriata. The 

 females are more inflated than in either of the northern forms, 

 and are usually more elongate. The laterals of A. walkcri are 

 heavier and lower, in proportion to the size of the shell, and 

 the main pseudocardinal of the right valve is more broadly 

 trigonal, with no tendency to be compressed. This last differ- 

 ence is most notable in the young specimens, as the juvenile 

 pseudocardinals of A. disca are both quite oblique and almost 

 lamellar, while those of walkeri are quite similar to those of 

 the adult in shape. The epidermis of walkeri is also thicker, 

 and the nacre attains a much more pronounced color (although 

 similar in shade) than in any specimens of A. disca that I have 

 seen. 



Actinonaias (Lcptodea?) tecomatensis (Lea) (1841).^ 

 Fourteen specimens, including odd valves, from the Rio San 

 Juan (H, vii, c). These specimens agree quite well with 

 typical tecomatensis. This species is very close to the more 

 northern^, tampicoensis (Lea) (1838). 



Measurements 



J ffi Q 



A. tampicoensis 80 75 35 (Simpson, 1914) 



A. tecomatensis 90 67 44 (Simpson, 1914) 



Mean, from Rio San Juan 88 70 45 



Extremes, ditto 66.5-96 66-76 40-49 



