86 MAZATLAX BIVALVES 



stices being pitted, tlie wliole shell is decussated by very fine 

 ratber distant concentric ridges, passing over and between the 

 ribs. The large ventral ribs are hoUowed in the interior of 

 the shell. Long. '03, lat. -04, alt. -02. 



Hah. — Mazatlan ; one valve in Spondyliis washings ; L'pool 

 Col. 



Genus ? TRAPEZIUM, 3Iegerh. 



Megerle von MiUdfeld, EnUourf kc. p. 68, 1811 : — Phil. Hand. 



Conch. J). 350. 

 Libitina, Schum. 1817. 

 Cj-pricardia, Lam. An. s. Vert. 1819- 



120. ? Trapezium — , sp. ind. 



Tablet 424 contains a puzzling little shell, too young to 

 identify even geuerically. It is shaped like the adult, not the 

 young of Cardita, but entirely without radiating ribs. SheU 

 transparent, subrhombic, irregular, anteriorl.y truncate, umbos 

 projecting, lateral ; valves smooth, with a few regidar distant 

 concentric epidermal ridges ; inside with a very stout j)rojecting 

 cardinal tooth, fitting between two small ones in the opposite 

 valve, and a posterior lateral tooth in each. Ligament external : 

 no trace of paUial sinus. Long. '02, lat. '04, alt. "03. 



Hah. — Mazatlan ; 1 pair, in shell washings ; L'pool Col.* 



Family CHAMID.E. 



This aberrant family is placed by Lam. and Woodw. between 

 Unio and Tridacna ; by D'Orb. between Spondylus and Ostrea ; 

 by Gray botw cen Saxioava and Cardita ; by Phil, between 

 Cardivuu and Lucina. In its adherent mode of life and u-regular 

 growth it resembles Ostrea ; while its two large adductors and 

 its siphons remove it far away. Both animal and shell have 

 relations with Isocardia (GlossidjB, G-ray) which was, not so 

 unnaturally as might appear at first sight, associated with it 

 by Liunanis. 



* Of the two species of Gouldia in the Cumingian Collection, G. cerina, 

 C -B. Ad. is confjenerif with the so called ' Circe ' minima, not with the Or. Pacitica 

 of the same author. Congeneric with the latter species and with G. varians are 

 CrassateUa Martiniconsis, D'Orb., intermediate in form between the two Mazat- 

 lan species, and Crassatella Guadaloupensis, D'Orb. the exact analogue of G. 

 I'acilica. For wliich group the goaeric name is to be retained, those who hane 

 aeon the diagnosis must decide. 



