4 GENUS THRACIA. 



1. THRACIA CORBULOIDES, DESK AYES. The Corbula-form- 

 ed Thracia 



(Collect. MASS.) MONTAGU, pi. 1, fig. I. 

 PI. II, %. 1. 



Th. testa ovato-transversa, grisea, incequivalvi, insequilatera, bisinuata, 

 umbonibus magnis, inferiore emarginato. 



Shell ovate, oblong, transverse, very inequivalve, inequi- 

 lateral, very much inflated, marked with irregular lines of 

 increase, entirely white, and covered with a grayish brown 

 epidermis ; the beaks of the valves very large, protuberant, 

 cordiforrn ; the right valve, which is the larger, has its beak 

 notched at its summit to receive the beak of the left valve. 

 The cardinal edge is delicate ; it presents upon its posterior 

 side a nympha, which projects within the valves, and which 

 receives a ligament partly internal and partly external. The 

 anterior extremity of the shell is obtuse, rounded ; the poste- 

 rior extremity truncated : this side is bounded outwardly by a 

 very prominent obtuse angle, which extends obliquely from 

 the beak to the lower part of the shell. Interiorly the valves 

 are white ; two muscular impressions are seen, very much 

 separated, the anterior of which is long and narrow ; the pos- 

 terior is rounded ; they are united by a palleal impression, 

 deeply notched posteriorly. 



Length 2 inches 5 lines. Width 1 inch 8 lines. 



This species, according to Montagu, may be considered a 

 borer, and is found in large quantities at Plymouth, imbedded 

 in rocks : it is this which probably occasions that diversity of 

 form which exists in this species. It is found also in the sand 

 in the harbor of Falmouth. Finally, it is found, also, in the 

 Mediterranean, where it appears to be larger than upon the 

 English coast, and where it is commonly found in the mud. 2 



2 Dr. A. A. Gould, of this city, found a specimen of this shell at Chel- 

 sea, in this state. Col. Totten, of Newport, Rhode Island, has met with 



