OSTEODESMACEA. MOLLUSCA. ANATINA. 47 



palleal marks indicated only by a more pearly appearance ; 

 they are far within the shell, and the latter has no well-marked 

 sinus. Length y\ inch, height -j-^^ inch, breadth y\ inch. 



It is found thrown upon the sandy shores of Cape Cod, Chelsea, 

 Lynn, and other similar localities. Its fragile structure is such as to 

 indicate that it could not live elsewhere than in quiet sand. In April, 

 1836, the beach at Chelsea was strewed with multitudes of very large 

 and mature ones. Since then I have found only an occasional 

 specimen. 



The ossiculum is almost always detached and lost. Sometimes, 

 when the valves are separated, it adheres to one of them, and then it 

 looks like the tooth of a Mya. When destitute of the ossiculum, if 

 reliance were placed upon the hinge alone, the shell would probably 

 be called an Amphidesma, or some undescribed genus. There is no 

 other shell on our coast, however, which presents the radiated wrinkles 

 of the epidermis, together with the pearly lustre, of this shell. 



The genus Osteodesma now embraces but three or four species. 

 One of these, the O. Norvegica of northern European seas, is very 

 similar to ours. But it is distinct. It grows to a much larger size, is 

 more inequilateral, more broadly truncate ; the base is less regularly 

 curved, and is covered by a much stronger and more opaque epi- 

 dermis. 



Young specimens are very thin, and have a horn-colored exterior, 

 and numerous thin, concentric ridges at the different stages of growth. 



If the valves are unequal, according to the definition of the genus, 

 the difference must be very slight. 



Genus ANATINA. 



Shell sub-equivahe, gaping slightly ; hinge with a prostrate, 

 spoon-shaped tooth in each valve to receive the cartilage, and a 

 small ossiculum resting in front of the teeth, usually removed with 

 the animal. 



Anatina papyra'cea. 



Shell thin, fragile, white, rounded-ovate, inequilateral, the 

 shorter side narrowed and truncated ; tooth narrow, directed ob- 

 liquely forwards. 



