84 MAZATIAN BIVAIVES 



altera valvd postico, altera antico magnis, longis ; cardinalibus 

 3-3, quarum medius altera valvd minimus, altera maximus ; im- 

 pressionihus 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 fossU Astartidee, it has concentric ribs either 

 near the umbo, all over the shell, or not at all. Sometmies m 

 forming lines of growth, it leaves a sharp projectmg ridge. 

 Sometimes it is of a somewhat regular Venus shape (especiaUy 

 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 margm 

 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 G. pacifica, but display 

 specific difi'erences in theu- comparative size. The surface 

 under a high power displays tlie strung-fig pattern, but on a 

 very much finer scale than in G. pacifica. Underneath the^outer 

 layer, the sheU is somewhat gramdose. 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. 

 Tlie smallest specimen measures lony. "03, lat. '035, alt. "015. 

 The largest „ „ >- ''^^ " '^^' " '^^• 



JTaS.— Mazatlan ; common in shell washings ; Lpool Col. 



Tablet 415 contains 1 pair and 12 valves ribbed.— 110, 4 valves 

 ribbed near the umbos.— 417, valves, nearly sniootn.— 418, 5 

 valves regularly rounded.-— 119, 5 valves elongated. 



Genus CAEDITA, Brug. 



118. Cardita Californica, Desh. 



Pro^. ZooZ. <Soc. 1852, p. 100. . ,^, ,0,- 1QQ .f 



? = Cardita affinis, Menke in Zeit. f. Mai. 184/, p. 188; et 



G-ould ms. : non Soio. 



As this is the northern form representing the more tropical 

 C. affinis (Soio. Proc. Zool. Soc. 1832, p. 195 ; v. also C.B.Ad. 

 Pan. Shells, V. 264) it is probal>ly the shell quoted as t,. athnis 



