Occasional Papers of the Museum of Zoology 22 
of the points mentioned in the original comparison might be 
applied to the latter, but I cannot believe that Simpson (1900) 
would ever confuse a species, even a male specimen, of this 
group with A. explicata. 
A. walkeri is smaller than typical disca and, in proportion 
to size, is also heavier than either disca or fimbriata. The 
females are more inflated than in either of the nerthern forms, 
and are usually more elongate. The laterals of A. walkeri 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 walker: 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 (Leptodea?) 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 A. tampicoensis (Lea) (1838). 
MEASUREMENTS 
é 
Sey net 
S es AS 
ete Rome ae 
= is] EG 2 @ 
oo én & 
Sy Sh) et I 
mt fa) 
A. tampicoensis 80 75 35 (Simpson, 1914) 
A. tecomatensis 90 = s«67 44 (Simpson, 1914) 
Mean, from Rio San Juan 88 70 45 
Extremes, ditto 66:5-96 66-76 40-49 
