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 stria, the impression of the linear composition of 
the ligament. he radiating rays of the exterior are broad and depressed, separated only 
by a thin, narrow line. 
2. PecruncuLvs prcussatus, J. Sowerby. Tab. XVI, fig. 7 a—d. 
PECTUNCULUS DECcUssATUS. 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. 
Spee. Char. P. testd suborbiculatd vel obtuse et irregulariter quadrangulari, equi- 
laterali, tenui, depressiusculd ; radiatim costellatd, concentricé striatd, decussatd; ared 
connevtis bipartitd; ared dentali arcuati, multidentatd ; umbonibus acutis ; marginibus 
integrts. 
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, sths; height, sths of an inch. 
Localities. Highgate (Wetherell), Bognor (Dixon), Basingstoke (Prestwich), Clarendon, 
Haverstock Hill (Zdwards). 
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 Zimopsis; this depressed ligament is strongly marked with lines at right 
