84, MAZATLAN BIVALVES 
altera valvé postico, altert antico magnis, iongis ; cardinalibus 
3-3, quarum medius altera valva minimus, alteré maximus ; im- 
pressionibus muscularibus subrotundatis, lined pallii a margine 
distantiore. 
Four or five well characterized species might easily be de- 
scribed from extreme forms of this variable little shell. Like 
one of the Crag fossil Astartidx, it has concentric ribs either 
near the umbo, all over the shell, or not at all. Sometimes in 
forming lines of growth, it leaves a sharp projecting ridge. 
Sometimes it is of a somewhat regular Venus shape (especially 
when young); sometimes it is narrowed and unusually pro- 
longed. Even in its most ribbed form, it differs from G. 
pacifica in being very much smaller, not so fiat, with umbos 
more spirally projecting, and with the anterior dorsal margin 
less coneave, as well as in having the ribs smaller, and closer. 
it has the general size and appearance of Astarte triangularis. 
The colour is generally whitish, with a large anterior brown 
stain. The teeth are on the plan of G. pacifica, but display 
specific differences in their comparative size. The surface 
under a high power displays the strung-fig pattern, but on a 
very much finer seale than in G. pacifica. Underneath the outer 
layer, the shell is somewhat granulose. About a score of pairs 
and more than 200 single valves were found on the large shells. 
it was not noticed by C. B. Adams at Panama, but it probably 
finds its way as far South, as I found it on specimens of Murex 
regius said to come from that place. 
The smallest specimen measures Jong. ‘03, lat. "035, alé. *015. 
The largest 29 29 39 “09, See O08, 33 “05. 
Hab.—Mazatlan ; common in shell washings; L’pool Col. 
Tablet 415 contains 1 pair and 12 valves ribbed.—416, 4 valves 
ribbed near the umbos.—417, 6 valves, nearly smooth.—418, 5 
valves regularly rounded.—419, 5 valves elongated. 
Gents CARDITA, Brug. 
118. Carpita Catirornica, Desh. 
Proce. Zool. Soc. 1852, p. 100. 
?=Cardita affinis, Menke in Zeit. f. Mal. 1847, p. 188; et 
Gould ms.: non Sow. 
As this is the northern form representing the more tropical 
C. affinis (Sow. Proc. Zool. Soc. 1832, p. 195; v. also C. B. Ad. 
Pan. Shells, p. 264) it is probably the shell quoted as C. affinis 
