32 INVERTEBRATA OF MASSACHUSETTS. 



beaks across the shell, growing wider and fainter in its progress ; 

 some fainter and broader radiations may also be seen in old shells, 

 when the epidermis is removed. Epidermis straw-colored, or 

 becoming fawn-colored, slightly wrinkled posteriorly. Within, 

 livid, smooth, and shining, becoming thickened by age. The red 

 stripe is visible within, and covered by a faint, rib-like thickening. 

 Hinge of two large, ascending teeth on each valve, one of 

 which, on the left valve, grows broad and is emarginate at tip. 

 Length J^ inch, height - inch, width 2 \ inch. 



This, like the preceding species, is occasionally found at New Bed- 

 ford, and other places in Buzzard's Bay. It is rather common about 

 Rhode Island. I have never met with a specimen north of Cape Cod ; 

 though, if our shell be identical with the S. frdgilis of British writers, it 

 is found on the Canada coasts. It differs from the fragile specimens 

 known in British Cabinets, simply in growing to a much larger size, 

 and becoming thick and strong, 



GENUS MACH^ERA, GOULD. 



Shell transversely oblong-oval, compressed, inequilateral, mod- 

 erately gaping; beaks minute. Hinge with three diverging car- 

 dinal teeth in the left valve, the middle one bifid, the third one 

 compressed, delicate, talcing the direction of the margin, or obso- 

 lete ; on the right valve two, entering between those of the opposite 

 valve. Within, usually crossed by a strong, longitudinal rib. 

 Muscular impressions joined by a deeply sinuous palleal line. 

 Ligament prominent. 



Animal not much larger than the shell ; lobes of the mantle 

 united for about half their length, the whole of their margin pecti- 

 nated with fleshy teeth from near the siphon to the hinge, except 

 where they pass over the foot ; similar bodies are also found 

 along their inner sub-margin, near the siphon. Labial palpi long, 

 extending quite across the foot, pointed. Branchiae extending to 

 the opening of the siphon and embracing about half the breadth 

 of the foot. Foot hatchet-shaped, dilating towards its extremity, 

 which is obliquely truncated. Siphons united to their tips, which 

 have scattered hairs. (WOOD-CUT of M. costdta, see page 24.) 



The above is the description of the animal of M. costata. It 



