84 MAZATLAN BIVALVES 



alterd valvd postico, alterd antico magnis, longis ; cardinalibus 

 3-3, quarum medius alterd valvd Minimus, alterd 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 Astartidse, 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 Gr. 

 pacifica in being very much smaller, not so flat, with umbos 

 more spirally projecting, and with the anterior dorsal margin 

 less concave, 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 Gr. 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 scale than in Gr. 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 long. '03, lat. '035, alt. '015. 



The largest '09, '08, '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. 



GENUS CAEDITA, Brug. 



\~ 118. CAEDITA CALIFORNICA, Desk. 



Proc. Zool. Soc. 1852, p. 100. 



? = Cardita affinis, MenJce in Zeit. f. Hal. 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 



