OSTEODESMACEA. MOLLUSCA. OSTEODESMA. 45 



the left or convex valve with three diverging teeth, the anterior 

 one much the longest and strongest, the middle one very delicate, 

 the third is rather a thickening of the posterior margin, with a 

 ledge in it for the reception of a tooth in the opposite valve. 

 Right or flat valve with two teeth, one short, triangular, strong, 

 directed across the shell, the other long, inclined to the posterior 

 hinge margin. Within iridescent ; muscular impressions rounded, 

 connected hy a series of about a dozen rough spots for the adhe- 

 sion of the mantle. Length ly^^ inch, height y\ inch, breadth 

 i inch, nearly. 



Found about the sandy regions of Cape Cod, and not unfre- 

 quently discovered adhering to oysters in the market. Dr. J. W. 

 Mighels of Portland, Maine, has taken it by dredging in Casco 

 Bay. Mr. Say found it as far south as Florida. 



This is a very curious shell, easily recognised by its pearly sub- 

 stance, its flat valve, and its upward curved tip. From all other spe- 

 cies yet described it is distinguished by the two lines bordering its 

 posterior hinge margin. Mr. Say has figured a small young speci- 

 men ; the rostrated tip is represented as too slender, as is also the 

 whole shell, and the pouch-like appearance of the posterior base is 

 not sufiiciently indicated for adult shells. There is often a good deal 

 of contortion and irregularity in the shape of the shell. 



Family OSTEODESmJiCE^. 



Shell transverse, inequilateral, incquivalve, fragile, somewhat pearly, slightly 

 gaping at one end; hinge ivith a thickening, or spoon-shaped process, to tvhich 

 the ligament is attached, usually supported ivithin by an ossiculum. 



There seems a propriety in separating from the family Myaria 

 some of the genera formerly included under it. The delicate 

 and pearly fabric of the shell, and the presence of the little ir- 

 regularly shaped bone resting against the hinge within, are well- 

 marked characters. These shells attain to a considerable size, 

 and live in the sand about low-water mark. 



Genus OSTEODESMA, Desh. 



Shell inequivabe, suhtriangular, fragile^ pearly ; hinge having 

 a narrow ledge loithin each valve, to which the ligament is attached^ 

 and against which adheres a four-sided ossiculum. 



