108 CONCHIFERA. DIMYAIRA, 



teral, with the valves close ; beaks decorticated ; one short 

 primary subifid tooth in each valve, with an oblong groove 

 divided by a strangulation ; ligament interior. 



U. oblonga, Lam. Shell fuscous yellow, wrinkled, rounded above, 

 longer than broad. 1 inch long Lam. v. 487- 



2. Shell not gaping at the sides ; ligament internal. 



Gen. 44. ERYCINA, Lam. 



Shell transverse, subinequilateral, equivalve, rarely gaping ; two 

 unequal primary diverging teeth, with a hollow interposed ; 

 lateral teeth two, oblong, compressed, short, re-entering ; li- 

 gament interior. 



E. cardioides, Lam. Shell ovate, orbicular, very small, with decus- 

 sated striae, the transverse striae remote ; longitudinal ones crowd- 

 ed. 4 lines long. Seas of New Holland Lam. v. 486. 



Fossil species of this genus have been found in France. 



Gen. 45. CRASSATELLA, Lam. 



Shell inequilateral, suborbicular or transverse, with the valves 

 close ; two primary subdiverging teeth, and a hollow at the 

 side ; ligament interior, inserted in the hollow of each valve ; 

 lateral teeth none, or obsolete. 



C. donacina, Lam. Shell ovate-trigonal, inequilateral, gibbous, 

 with fine transverse striae ; umbones smooth ; epidermis reddish 

 brown. Inhabits seas of New Holland. Lam. v. 481. 



C. sulcata, Lam. Shell ovate-trigonal, widely inequilateral, gib- 

 bous, with transverse sulcated plaits; posterior side produced, 

 angular. Inhabits seas of New Holland. Lam. v. 481. 



Fossil species of this genus are found in France, and in Britain in the London clay. 



3. Ligament internal ; shell gaping at the sides. 



Gen. 46. MACTRA, Lam. Lin. 



Shell transverse, inequilateral, subtrigonal, slightly gaping at 

 the sides, with the beaks protuberant ; one primary compres- 

 sed tooth in each valve, and an adjacent heart-shaped cavity ; 

 two lateral teeth near the hinge, re-entering ; ligament in- 

 terior. 



This genus is numerous in species. They are all marine shells, of a trigonal 

 form, slightly protuberant on the sides, and smooth, wrinkled or furrowed trans- 

 versely. At one of the sides of the shell the animal protrudes two tubes formed 

 of its mantle, and at the other a muscular compressed foot. A few fossil species of 

 this genus are found in England. 



M. Spengleri, Lin. Shell trigonal, smooth, with the anterior slope 

 flat, and a transverse crescent-shaped gap ; umbones distant. 2 

 inches long. Inhabits sea at Cape of Good Hope. Chem. vi. 

 pi. 20, fig. 199-201. 



M. stultorum, Lin. Shell subtriangular, diaphanous, smooth, ob- 

 soletely radiated, and the umbonal region gibbous ; inside pur- 



