BIVALVIA. © 97 
angles to the dental margin (fig. 7, c, d), showing the linear fibres of which it is composed, 
these being the more durable portion, are alone remaining. Fig. 14, Tab. XIX, is the 
representation of a young individual from Clarendon; it is of a rather more elongated 
form than the generality of the larger specimens, but its peculiarity is in the area for con- 
nexus, where it shows a bipartite character precisely resembling that which is considered a 
generic distinction in Zemopsis ; in this young shell the triangular cavity is not only small, 
but it is comparatively much less than in the adult shell. The connexion between the 
two genera in the immature state appears so close as not to permit of generic separation, 
showing, as in many other animals, a very near relationship in the early part of life, diverging 
by the increase of age. 
Some of these young shells have the rays upon the exterior fewer and more prominent, 
resembling those upon P. de/etus, the intermediate rays being small and scarcely per- 
ceptible ; they are not peculiar to the Clarendon specimens, but may be seen also on young 
shells from Haverstock Hill. Casts resembling this species have been found at Sheppey. 
3. Precruncunus pEeLrtus, Solander. Tab. XVI, fig. 3, a, d. 
Arca DELETA. Solander, in Brand. Foss. Hanton., p. 97, pl. vil, fig. 97, 1766. 
PEcTUNCULUs costatus. J. Sow. Min. Conch., t. 27, fig. 1, 1813. 
— peLntus. Morris. Catal. Brit. Foss., p. 219, 1854. 
Spec. Char. P. testdé orbiculatd, converd, vel regulariter lenticulatd ; radiatim 
costatd ; costis equalibus angustis, acutis ornatd ; concentricé striata ; striis creberrimis ; 
costis aliquando tuberculats ; umbonibus subclevatis, recurvis; cardine valdé arcuato, 
multidentato. 
Shell orbicular, convex, or lenticular, radiately costated ; ribs equal, sharp, and narrow; 
concentrically striated lines of increase numerous, close, ribs sometimes tuberculated; beaks 
slightly elevated, recurved ; hinge-line with numerous teeth. 
Diameter, 13ths of an inch. 
Locality. Barton. 
This is an abundant shell at Barton. I have not seen it from any other locality. 
Some specimens are almost smooth, or at least are covered with only depressed rays, 
without the appearance of abrasion, others are beautifully ornamented with narrow sharp 
ribs, varying from twenty-five to thirty-five; and these are, in very well preserved specimens, 
covered with tubercles produced by the prominent lines of growth. ‘Ihe dental area is 
well furnished with a continuous line of teeth varying from twenty to thirty. The 
margin of the valves is regularly denticulated, and these are not the extension of the ribs, 
but appear to be quite independent. 
A shell found by Herr A. von Koenen at Lattorf appears to agree with the smooth 
* variety of this species. 
