52 



CIIAPTEK VI. . 



DESCRIPTION OF BIVALVE SHELLS, CONCLUDED. 



Pachydesma (Trigona) crassatelhides, Corn*., 

 Triangle-shell, Fig. 1, PI XIV, has a ponderous 

 shell, sometimes growing to great size. The 

 pair of valves from which the figure was drawn 

 are five and one half inches in length, and weigh 

 over a pound. They are very thick and solid, 

 beautifully white internally, with purple muscu- 

 lar impressions. The outside is yellowish white, 

 partly covered by a glossy epidermis, and some- 

 times marked by radiating stripes. The edges of 

 the shell are smooth and rounded; the hinge 

 teeth are very strong, consisting of three cardin- 

 al and one lateral ; the ligament is external and 

 bulged ; the pallial sinus, small. This great 

 mollnsk is sometimes dug up from its home and 

 used for focd. 



Another strong, thick shell, though not so 

 large as the last, is Saxidomus Nuttallii, Conr., 

 Fig. 2, PI. XIV, Nuttall's Eock-clam. It some- 

 times grows to a length of four inches. The out- 

 side is of a dirty white, and is marked by 

 numerous irregular, concentric ridges. The in- 

 terior is wdiite, and the thick part below the 

 external ligament is translucent like agate. The 



