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 m much 
finer concentric lines. 
C. Californica begins life in an irregular way without ribs, 
as a tiny white creature with concentric strie. 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, subtrigonal, and nearly equilateral. At this 
time there is alarge, 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, lat. 1°38, alt. °5. 
Hab.—Gulf of California, Mus. Cuming.— Mazatlan ; extremely 
rare adult, rare jun., nestling in crevices of Chamez, Spondyh, 
Ostree 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 Discina Cumingii, attached.—422, an adult 
specimen. 
Gents VENERICARDIA, 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. a 
