MACTRACEA. MOLLUSCA. MESODESMA. 57 



same, and one or two more have been found in the Pacific ; and these 

 are all the species at present known. 



Genus MESODESMA, Desh. 



Shell triangular, co7npressed, thick, generally closed, hinge a 

 spoon-shaped tooth in each valve, ivhich receives the cartilage, and 

 a simple lateral tooth on each side, ivithout a V tooth. 



Mesode'sma arcta'ta. 



Shell sub-triangular, very inequilateral, truncated before, smooth 

 and covered by a shining ye/low epidermis ; lateral teeth straight, 

 striated. 



Figure 39. 



State Coll., No. 223. Soc. Cab., No. 1737. 



Mactra arctuta, Conrad ; Journ. A ad JYut. Sc, vi. 257, pi. 11, f. 1. 

 Mactra deaiiruta, Conrad ; Amer. Mur. Conch., 59, j)l. 14, f. I. 

 Mactra subtriangul'ita, Wood ; Index, SuppL, pi. 1, f. 10. 



Shell sub-triangular, wedge-shaped, thick and strong, very 

 inequilateral, the posterior side very short, forming the base of 

 the wedge, its lower portion truncated ; the upper and anterior 

 edge is a straight line, the anterior extremity narrowed, regularly 

 rounded, as is also the base ; beaks very slightly elevated, not in- 

 clined to either end ; a sharp ridge passes from them to the 

 hinder and lower angle ; surface with occasional rounded ridges 

 at the lines of growth, covered with a thin, golden-yellow epider- 

 mis, reflecting a metallic lustre ; hinge composed of a very deep, 

 spoon-shaped cavity for the cartilage, a long V tooth opening at a 

 very acute angle, and on each side a straight lateral tooth, par- 

 tially double in the right valve, their articulating surfaces striated, 

 the posterior tooth much the shortest ; muscular impressions well 

 defined, connected by the simple palleal impression which has a 

 sinus behind, of about the size of the rounded muscular impres- 

 sion at its side. Length 1-j inch, height 1 inch, breadth ^1 inch. 



Found abundantly at Plumb Island, below Newburyport ; vast 

 numbers are also thrown upon the outer side of Cape Cod ; and a 

 few specimens have been taken from fish caught ofi' Nahant. At 

 Nantucket it is rare. 



