4 GENUS THRACIA. 
1. THRACIA CORBULOIDES, Desnayes. The Corbula-form- 
ed Thracia 
(Collect. Mass.) Mownragzu, pl. 1, fig. 1. 
Pl. UI, fig. 1. 
Th. testa ovato-transversa, grisea, inequivalvi, inequilaterd, 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, 
cordiform ; 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 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 
