MAZATLAN BIVALVES 85 



by Menke in his list of Melchers' Mazatlan shells, and by 

 Gould as collected by Lieut. Green at Guaymas. I have C. 

 affinis from S. W. Mexico ; but all, even the most minute, 

 specimens from the Mazatlan collection belong to this species. 

 C. Californica differs from C. affinis (as far as my few speci- 

 mens enable me to judge) in being rather less transverse, less 

 incurved at the anterior ventral part, less narrow and projecting 

 anteriorly, with the ribs rounder, larger, and covered with 

 very large scaly protuberances at the posterior end. The 

 colour also is much redder, and the epidermis lies in much 

 finer concentric lines. 



C. Californica begins life in an irregular way without ribs, 

 as a tiny white creature with concentric striae. The ribs how- 

 ever on appearing soon become very large and well armed. 

 The creature then looks like a tiny Tridacna, with extremely 

 projecting umbos, sub trigonal, and nearly equilateral. At this 

 time there is a large, distant, anterior lateral tooth. (There 

 seems a general tendency among bivalves, in their earliest 

 stage, to an abnormal development of the anterior portion. 

 Pis the foot then more active than the siphons. Gradually the 

 posterior part grows, the colour appears, and the anterior 

 tooth draws nearer (proportionally) to the hinge. There are 

 generally one or two fewer ribs developed in -the young than in 

 the adult. The smallest specimen is '03 across ; the largest 

 measures long. '78,Jat. 1*38, alt. *5. 



JTab. Gulf of California, Mus. Cuming. Mazatlan ; extremely 

 rare adult, rare jun., nestling in crevices of Chamse, Spondyli, 

 Ostrese andPerna; L pool Col. ? Guaymas, Lieut. G-reen- 

 Tablet 420 contains 3 minute valves and 2 pairs jun. 421, a 

 young specimen in situ on the back of a Perna, on which is the 

 lower valve of Discina Cumingii, attached. 422, an adult 

 specimen. 



GENUS VENEEICAEDIA, Lam. 



119. ? VENERICARDIA , sp. ind. 



Tablet 423 contains a minute valve, whose characters are all 

 those of Cardium except the hinge. This however presents a 

 triangular cardinal tooth, with pits for the 2 opposite ones, and 

 an indistinct, closely approximate anterior lateral. There is 

 no trace of posterior tooth. The outside is ribbed almost 

 exactly as in Cardium graniferum ; but instead of the inter- 

 Nov. 1855. * 



