Description of some New Shells. • 367 



3d. There is, at least so far as my examination has extended, a 

 constant difference in the form of the callus at the hinge. The un- 

 der side, arched and forked to embrace the posterior part of the 

 anterior muscle, has, in our species, a less radius of curvature, and 

 the hinder portion points obliquely forward, while in S. velum ? it points 

 across the shell. 



An enlarged sketch of the anterior end of S. velum ? is given fig. 1 . k. 



Venus gemma. 



Fig. 2. a. b. c. are enlarged — d. natural size. 

 Shell sub-rotund, nearly equilateral, concentrically furrowed, glos- 

 sy ; anterior portion and basal margin, both within and without, white 

 or pale reddish-violet, remainder reddish purple, darker at and near the 

 superior and posterior margins: no lunule: beaks small, incurved, 

 separate, generally eroded : teeth divergent, the medial tooth of each 

 valve stout triangular, the anterior tooth of the right, and the posterior of 

 ihe left valve, thin and not easily distinguished : inner margin crenulate. 



Length, 0*15 of an inch. 

 The length being represented by 13 

 the breadth will be 12 



and the diameter, 6. 



Inhabits the coast of Massachusetts and Rhode Island. 

 I found this beautiful little shell, first, on the beach at Province- 

 town, Cape Cod, (Mass.) ; it has since been found in Newport harbor. 

 The largest specimen I have seen among many hundreds, is bare- 

 ly ^ of an inch in length; the more common length being about T ' ? 

 of an inch. 



It is often much eroded on the disks, and then the color is bluish 

 purple. 



Modiola 



Fig. 3.-6./. 



Shell obliquely oval, or sub-ovate, close, somewhat inflated, con- 

 vexity generally rising regularly and gradually from the margins, 

 sometimes an indistinct umbonial ridge reaches the posterior-basal 

 margin : valves concentrically slightly wrinkled and minutely striated, 

 and rayed with very numerous rounded ribs proceeding in all di- 

 rections from the apex, increasing in number on the disk, and rather 

 deeply cut by lines of growth : epidermis dull brownish-yellow, be- 

 neath which the surface is white : substance of the shell thin, translucent : 

 beaks rather prominent, separate, recurved, decorticated : all the inner 

 margin notoccuoied bv the lie-ament crenulate : within white and oearlv. 



