MAZATLAN BIVALVES 85 



by Menke ia liis list of Melcliers' 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, 

 specmiens from the Mazatlan collection belong to this species. 

 C. CaUfornica diflfers 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 

 nner concentric lines. 



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

 as a tmy white creature with concentric stria;. The ribs hoAvl 

 ever on appearing soon become very large and well armed. 

 The creature then looks like a tiny Tridacna, with extremely 

 projecting umbos, subtrigonal, 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. GraduaUy the 

 posterior part grows, the colour appears, and the anterior 

 tooth draws nearer (proportionaUy) 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, lat. Vm, alt. "5. 



.fia/j.— Gulf of California, Mus. Cuming.— Mazatlan ; extremely 

 rare adult, rare jim., nestling in crevices of Chama;, Spondyli, 

 Ostreje and Perna ; L'pool Col.-? Guaymas, Lieut. Green. 

 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 Discma Cumbgii, attached.— 422, an adult 



specimen. 



Genus VENERICAEDIA, Lam. 



119. P Venericaedia , sp. ind. 



Tablet 423 contains a minute valve, whose characters are all 

 those of Cardium except the hinge. This however presents a 

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

 an indistmct, closely approximate anterior lateral There i8 

 no trace of posterior tooth. The outside is ribbed almost 

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

 JVov. 1855. 



