96 EOCENE MOLLUSCA. 



Shell suborbicular, slightly ovate, tumidly lenticular, nearly equilateral and sym- 

 metrical ; covered with obsolete radiating ridges, and concentrically striated ; beaks short 

 or depressed; area of the connector large, dental margin curved, teeth few and large; 

 margins crenulated. 



Diameter, 2 inches. 



Localities. Bognor {Sowerby), Reading {Morris). 



This is a long and well-known shell at Bognor. where it has been found in abundance, 

 with the valves generally united, and their ventral margins closed ; the area for the liga- 

 ment is rather wide, and ornamented with about half a dozen diverging depressed 

 lines, and these oftentimes bear vertical striae, the impression of the linear composition of 

 the ligament. The radiating rays of the exterior are broad and depressed, separated only 

 by a thin, narrow line. 



2. Pectunculus decussatus, /. Sowerby. Tab. XVI, fig. 7 a — d. 



Pectunculus decussatus. J. Sow. Min. Conch, t. 27, fig. 1, 1812. 



— — Id. in Dixon's Geol. of Suss., p. 116, t. 14, fig. 7, 1850. 



_ _ Smith. Strata. Identif., t. 11, fig. 10, 1816. 



Spec. Char. P. testa suborbiculatd vel obtuse et irregular iter quadrangular/', cpqui- 

 laterali, tenui, depressiusculd ; radiaiim costellatd, concentrice striata, decussatd ; area 

 connexus bipartitd ; area dentali arcuatd, midtidentatd ; umbonibus acutis ; marginibus 

 integris. 



Shell suborbicular or obtusely and irregularly quadrangular, equilateral, thin, and 

 somewhat depressed ; radiately striated, and decussated by lines of growth ; area of the 

 connector bipartite ; dental margin curved and well filled with teeth ; beaks sharp, margins 

 smooth. 



Length, fths ; height, fths of an inch. 



Localities. Highgate [Wetherell), Bognor {Dixon), Basingstoke {Prestwich), Clarendon, 

 Haverstock Hill {Edwards). 



This is abundant at Highgate, and Mr. Sowerby has figured a specimen from Bognor, 

 where, I believe, it is rare. The specimens from Highgate are generally in a good state of 

 preservation except at the umbones, nine tenths at least are there broken. The outline of 

 this species is more quadrangular than in the generality of the genus, especially at the 

 siphonilateral margin, and the shell is rather longer than it is high. The surface is 

 prettily ornamented by the lines of growth, decussating the rays, by which they are made 

 slightly nodulous ; the radiating lines are occasionally distant, with one to three interme- 

 diate or smaller rays. The radiations of the mantle are generally impressed upon the 

 interior of the shell, and the impressions of the adductors are very large. The area for the 

 connector is somewhat peculiar, having a large obtusely angular depression, and it is bipar- 

 tite, like that of Limopsis ; this depressed ligament is strongly marked with lines at right 



