86 MAZATLAN BIVALVES 
stices being pitted, the whole shell is decussated by very fine 
rather distant concentric ridges, passing over and between the 
ribs. The large ventral ribs are hollowed in the interior of 
the shell. Long. 03, lat. °04, alt. *02. 
Hab.—Mazatlan ; one valve in Spondylus washings; L’pool 
Col. 
Genus P TRAPEZIUM, Megerle. 
Megerle von Mihi feld, Entwurf &e. p. 68, 1811 :—Phil. Hand. 
Conch. p. 350. 
Libitina, Schum. 1817. 
Cypricardia, Lam. An. s. Vert. 1819. 
120. ? TRapEzIUM ——— , sp. ind. 
Tablet 424 contains a puzzling little shell, too young to 
identify even generically. It is shaped like the adult, not the 
young of Cardita, but entirely without radiating ribs. Shell 
transparent, subrhombie, irregular, anteriorly truneate, umbos 
projecting, lateral; valves smooth, with a few regular distant — 
concentric epidermal ridges ; inside with avery stout projecting 
cardinal tooth, fitting between two small ones in the opposite 
valve, and a posterior lateral tooth in each. Ligament external : 
no trace of pallial sinus. Long. ‘02, lat. 04, alt. °03. 
Hab.—Mazatlan ; 1 pair, in shell washings ; L’pool Col.* 
Famity CHAMID. 
This aberrant family is placed by Lam. and Woodw. between 
Unio and Tridaena ; by D’Orb. between Spondylus and Ostrea ; 
by Gray between Saxicava and Cardita; by Phil. between 
Cardium and Lucina. In its adherent mode of life and irregular 
growth it resembles Ostrea ; while its two large adductors and 
its siphons remove it far away. Both animal and shell have 
relations with Isocardia (Glosside, Gray) which was, not so 
unnaturally as might appear at first sight, associated with it 
by Linneus. 
* Of the two species of Gouldia in the Cumingian Collection, G. cerina, 
C. B. Ad. is congeneric with the so called ‘ Circe’ minima, not with the G. Pacifica 
of the same author. Congeneric with the latter species and with G. varians are 
Crassatella Martinicensis, D’Orb., intermediate in form between the two. Mazat- 
lan species, and Crassatella Guadaloupensis, D’Orb. the exact analogue of G. 
Pacifica. For which group the generic name is to be retained, those who have 
seen the diagnosis must decide. 
