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 5. 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. 6. 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 
the 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 3, of an inch in length; the more common length being about {5 
an inch. 
I is often much eroded on the disks, and then the color is bluish 
le. 
Modiola glandula. 
Fig. 3.—e. fi g. 
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 not occupied by the ligament crenulate : within white and pearly. 
