MESODESMA. 347 



Mesodesma Jaurcsu,DE Joannis, Mag. de Zool. 1834, Moll. pi. .54 Gould, 



Invert. Massach. p. 58, f. 38. — Dekay, New York Moll. 

 p. 231. 

 „ doauratum, Hanl. Recent Shells, p. 39, suppl. pi- 1, Mactra, f. 9. — 



Brit. Marine Conch, p. 53. 



Somewhat triangularly subovate, thick, heavy, opaque, very 

 inequilateral, moderately convex behind, compressed in front ; 

 valves, when young, perfectly smooth and covered with a shining 

 yellow epidermis ; when aged, antiquated, and the epidermis 

 olivaceous or dusky brown, beneath which the surface is whitish, 

 or pale reddish brown. The anterior side is nearly double the 

 length of the posterior, and tapers to a rounded extremity, the 

 ventral margin, which is straightish near the middle, strongly 

 ascends anteriorly in an arcuated sweep ; the hinder extremity 

 is most bluntly subcuneiform, the tip being broad and slightly 

 rounded. The inclination of the front dorsal edge is very trifling, 

 that of the hinder is much greater, but still moderate ; the con- 

 vexity of both of them is so slight as scarcely to be apparent. 

 The beaks are not greatly elevated, but are rather acute when 

 not eroded, and do not lean to either side ; there is a slight flat- 

 tening of the dorsal edge in front of them, but no lunule. The 

 interior is more or less white, and the margins entire ; the carti- 

 lage pit is broad, deep, and subtriangular, shelving downward 

 so that its base is below the level of the inferior edge of the hinge 

 margin, and appressed upon the subumbonal region ; its lower 

 edge is convex. The lateral teeth are remarkably strong and 

 solid, and are more or less coarsely sculptured with raised perpen- 

 dicular striae ; both of them are approximate, and the anterior is 

 distinctly the more produced. A small semicircular posterior 

 sinus alone prevents the pallial impression from being simple. 



The full dimensions assigned to it by Dr. Gould are an inch 

 and three-quarters in length, by a little more than an inch in 

 breadth. Our own and Dr. Turton's examples, are far less. The 

 American author, whose opportunities of examining numerous 

 specimens were of course superior to ours, remarks, that, on the 

 anterior edge of the deep spoon-shaped cartilage-pit " is the ves- 

 tige of a short, widely diverging, V-shaped tooth, which will 

 seldom be found, as it is scarcely possible to open the valves with- 

 out destroying it." 



