BIVALVIA. 39 
I have seen this from one locality only. The solitary line spoken of by Mr. 
Sowerby, in ‘ Min. Conch.,’ as generally running down the centre of the interspaces, is a 
character not constant; in some it is not visible, while in others this space is occupied 
with several lines. The spaces between the ribs are not quite so wide as the ribs 
themselves, and these show fine curving and radiating striz. The keel in some 
individuals is large and strong, giving an almost angular form to the rib; but in 
general this keel is merely a sharp line upon the otherwise rounded costz, closely ap- 
proaching P. reconditus. The auricle on the pedal side is slightly the larger of the two, 
and in the right valve this auricle is costated and sinuated beneath, with three or four 
denticulations upon the margin of the shell. 
3. Pecren corneus, J. Sowerby. ‘Tab. IX, fig. 7, a—d. 
Prcrren corneus. J. Sow. Min. Conch., t. 204, 1818. 
— — Id. in Dixon’s Geol. of Sussex, t. 4, fig. 6, 1850. 
_ — Nyst. Coq. Foss. Belg., p. 299, pl. 23, fig. 1 a, 6, 1843. 
— — Morris. Cat. Brit, Foss., p. 178, 1854, 
Spec. Char. P. testd tenuissimd, orbiculato-depressd vel plano-lenticulata, equilaterali, 
levigatd, vel obsoleté striatd ; strits irregularibus ; umbonibus acutis ; auriculis subequa- 
libus, rectangulis, superné prominentibus, tenuissimé striolatis. 
Shell very thin, orbicular, depressed, or of a flattened lenticular form ; smooth, or with 
fine, nearly obsolete striae ; beaks sharp; auricles nearly equal, rectangular ; generally 
with fine radiating strize. 
Diameter, 2% inches. 
Localities, Bracklesham, Bramshaw, Brook, Stubbington (Edwards), Cuffell (Prest- 
wich); var. corneolus, Highgate (Wetherell). 
Belg., Les sables de Lacken, Jette, Forét, St. Gilles (ys). 
This species is abundant at Bracklesham, but, on account of its extreme tenuity, is not 
very common in cabinets. Our shell, which is smooth to the naked eye, exhibits under a 
common hand-glass fine diverging strie, most distinct near the margins of the shell, and in 
well-preserved specimens they are plainly visible all over; it is also rayed irregularly on 
the inside. ‘The auricles are plain and smooth, except under a magnifier, when the same 
kind of fine radiating strie may be seen as are upon the shell. ‘There are at the 
base of the auricles, on the interior, two prominent diverging teeth, those in the right 
valve being higher and smaller than in the other, and the adductor muscle-mark is very 
conspicuous, generally of a darker colour; this is large and rounded, and in the right 
valve, which is the deeper or more convex one, it is higher in the shell or nearer to the 
hinge than in the left or flatter valve, where it is more in the centre, giving a more 
powerful action to that valve, and the mark of the mantle is visibly impressed upon the 
interior. The shell is nearly orbicular, but sometimes the diameter is greater longi- 
