498 



antica costata et imbricata, margine ventrali leevij infiato, margiue 

 dorsali confluente, usque ad umbones reilexo^ tumido ; parte pos- 

 tica l;evi, subtruncata, ad termiuum appeiidice cyathiforini lateribus 

 subciuadratis. 



This common British species is rather cylindrical in form, 

 enclosed at the anterior by a thin test, which, from the ventral, is 

 continued over the dorsal margins in a kind of double ridge. 

 Close to the umbones, this ridge is divided transversely, so as to 

 form a pair of separate plates. At the posterior termination is a 

 cup-shaped appendage, which is rather quadrate in form. 



Fotmd in various parts of Great Britain. 



38. Pholas clausa (pi. cvii. f. 74, 75), Gray. P. expla- 

 nata, Spengler'^ 



Ph. testa clausa, subovali, ventricosa, omninb striis radiatis 

 et costis moniliformibus concentricis ornata ; margine dorsali pos- 

 tice sufflato, margine dorsali antice valide expanso et sufflato, ad 

 umbones transverse inciso, super laminas transversas duplicatas ex- 

 pansas decumbente ; ad terminum posticum subattenuata, epider- 

 mide nigricante, lobis quatuor inflatis, tiini integumento cyathi- 

 formi expanso, ad latera emarginato. 



This species is not divided by a central grove, but is reticulated 

 by fine raised strise crossing concentric knobbed ribs. The dorsal 

 margins are swelled, the anterior swelled out and turned back 

 over the umbones, where they are transversely divided and placed 

 upon a transverse platform, which is wide and acuminated, and 

 also divided in the centre. The cup is expanded, emarginated at 

 the sides, and surmounted by four rounded and swelled lobes. 



River Gambia. Capt. Shaw. 



39. Pholas tridens (pi. cvi. f. 60, 61). Talena tridens. 

 Gray, Brit. Mus. Cat. 



Ph. testa clausa, subovali, canaK serrato divisa; parte antica 

 costata et lineata, margine ventrali inflato, cum margine dorsali 

 confluente ; margine dorsali tumido, diviso, ad umbones transverse 

 inciso ; parte postica concentrice lineata, ad terminmn tumis arcu- 

 atis duabus, ad marginem posticum integumento cyathiformi ; 

 cyatho intiis callo tripartito. 



Resembling Ph. papyracea (f. 66) in some degree, but more 

 tapering posteriorly, and the cup is surmounted by an arcuated 

 tumidity on each side, which within is marked by a tripartite 

 callosity. 



Monte Clu'isto. Cuminf/. 



