52 
CHAPTER VI. 
DESCRIPTION OF BIVALVE SHELLS, CONCLUDED. 
Pacliydesma (Trigona) crassatelloides , 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 
moilusk is sometimes dug up from its home and 
used for foed. 
Another strong, thick shell, though not so 
large as the last, is Saxidowus Nuttallii, Conr., 
Fig. 2, PI. XIV, Nuttall’s Rock-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 white, and the thick part below the 
external ligament is translucent like agate. The 
