418 MOLLUSCA. 



ing from the marginal impressions of their mantle, their tubes must 

 be very large(l). 



CoRBULA^ Brug. 



Similar in form to the triangular Cytherese, or cordate; but a single 

 stout tooth in the middle of each valve, corresponding to the side of 

 its antagonist. The ligament is internal; the tubes must be short, 

 and the valves but rarely equal(2). 



The fossil species are much more numerous than the living 

 ones(3). 



Some of them live in the interior of stones(4). 



Mactra, Lin. 



The Mactrse are distinguished from the other Testacea of this family 

 by their ligament being internal, and lodged throughout in a trian- 

 gular depression, as in the oysters; they all have a compressed foot 

 fitted for crawling- In the 



Mactra, Lam., 



Or the Mactrae properly so called, the ligament is accompanied 

 to the left valve, before and behind, by a projecting plate which is 

 received between two others on the right one. Close to the ligament, 

 near the lunule, is a little plate en chevron. The tubes are united 

 and short(5). 



Some of them are found on the coast of France. 



In the Lavigxons, the lateral plates are almost effaced, but a sin- 

 gle small tooth is observable near the internal ligament; there is 

 also a second and internal ligament. The posterior side of the shell is 



(1) Ven. lapicida, Chemn., X, 172, 1664, and the RupelljiHia of M. Fleriau de 

 Bellevue; — Ven. perforans, Montag., Test. Brit. pi. iii, f. 6; — Donax irus? 

 Chemn., VI; xxvi, 270. 



(2) See Encyc. Method., Vers, pi. 230, f. 1,4, 5, 6. 



(3) Corbula gallica; — G. complanata,- — G. omhonella, Desh., Coq. Foss., des Env. 

 de Paris, 1. 1, pi. 7, 8, 9. 



(4) Venus monstrosa, Chemn., VII, 42, 445 — 446. 



(5) After abstracting the Lavignones and Lutrarix, the genus Mactra of Gme- 

 lln may remain as it is; the species, however, are far from being well distinguished. 

 Add, Mya australls, Chemn., VI, iii, 19, 20. 



The EnTcixTE, Lam., are neighbours of the Mactrx, and are but badly cliarac- 

 terized. Sec Ann. du Mus., IX, xsxi, and Deshayes, Coq. Foss., I, vi; part of 

 them, perhaps, belong to the Crassatellae. The Amphidesmvb, Lam., or Ligulje, 

 Montag., appear to approach the Mactrze, but they are too imperfectly known to 

 have any distinctive character assigned to them. 



