CONCHOLOGY. 65 



FAMILY XII. 



Trigonacea. Two genera. 



1. Trigonia. One species. 



Supposed 'to be in very deep places in the sea; it is a 

 strong, beautiful, pearly shell, sub-trigonal or sub-orbicular; 

 thick, regular, equivalve, inequilateral ; summits slightly 

 prominent, recurved, anterodorsal ; hinge complex, dorsal, 

 dissimilar; two large oblong teeth laterally compressed, 

 joined angularly under the summit, strongly furrowed on the 

 right valve, penetrating into two excavations of the same 

 form, equally furrowed on the left valve ; ligament exterior 

 and marginal ; two distinct muscular impressions. 



T. pectinata. The pectinated Trigonia. PL 11, fig. 4. 



Species sub-orbicular, with radiated or divergent, promi- 

 nent, and somewhat scaly ribs ; inside pearly ; margin, cren- 

 ulated. 



2. Castalia. One species. 



This genus is found in fresh waters, and differs from the 

 Trigonia in the number and position of the lamellar teeth. 

 The substance of the shell is pearly. 



Shell sub-trigonal, equivalve, inequilateral ; umbones ero- 

 ded, covered with epidermis, and inflected anteriorly ; hinge 

 with two lamellar teeth, transversely striated, one of them 

 posterior, distant, and shortened, the other anterior, length- 

 ened, and lateral ; ligament exterior. 



C.amhigua. The ambiguous Castalia. PI. 11, fig. 5. 



Short, sub-trigonal ; umbones truncated ; longitudinally 

 ribbed, with distant transverse striae ; covered with epider- 

 mis, under which the shell is of a pale chestnut brown, in- 

 side pearly ; the lamellar and prse-apicial teeth are well 

 marked, more regular, and all striated perpendicular to their 

 length. 



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