84 INVERTEBRATA OF MASSACHUSETTS. 



Pennant. From this it is easily distinguished by its epidermis, and 

 also by wanting the purple border along the interior margin of the 

 shell. 



It is subject to very little variety. Sometimes the surface has a 

 series of concentric ridges, and sometimes the beaks are unnaturally 

 elevated and curved. The old shells have a very dark epidermis, and 

 are generally much decorticated. The young are of a light fawn- 

 color, with darker and lighter zones ; and the wrinkles, being much 

 raised, and very regular, give the surface a very pretty appearance. 

 The rougher surface, greater proportionate length, rounded form, and 

 want of a lunule, distinguish them from Astdrte quadrans of a similar 

 size. From Astdrte compressa, they differ little. 



They are obtained of every size, from one fourth of an inch to 

 four inches in length. 



GENUS CYTHER^A, LAM. 



Shell inequilateral , rounded ; hinge ivith four diverging teeth in 

 one valve, one of which is separate from the others, and three in the 

 other valve ; no lateral teeth. 



CYTHERE A CONVEXA. 



Shell oval, tumid ; lunule heart-shaped ; beaks prominent and 

 recurved ; color chalky-white. 



FIGURE 49. 

 State Coll., No. 198. Soc. Cab., No. 2332. 



Cytherea convexa, SAY ; Journ. Acad. Nat. Sc., iv. 149, pi. 12, f. 3 ; vi. 261. 



Shell oval, rather thin, valves very convex, color a dead-white 

 or with a rusty tinge, usually chalky ; inequilateral, beaks ele- 

 vated, curving forwards, in front of which is a well-marked heart- 

 shaped lunule ; anterior side about half the length of the posterior, 

 compressed and somewhat pointed ; regularly rounded behind and 

 at base ; surface marked with coarse lines of growth, which are 

 most regular anteriorly ; ligament long, rather sunken ; within 

 milk-white, polished ; impressions superficial. Length 1 inch, 

 height 1J inch, breadth 1 inch nearly. 



A few specimens have been found on Chelsea Beach, but it is 

 rare. It has been found about Rhode Island by Colonel Totten ; 



