82 MOLLUSCA FROM THE CRAG. 



the Paris Basin ; there are, however, marked differences between the two, sufficient to 

 keep them specifically distinct. The older shell has a crenulated margin, with other 

 characters of minor importance by which it may also be distinguished, while the 

 Crag one has its margin perfectly smooth. A shell in the Cabinet of Mr. D'Urban, 

 found in the Eocene Formation at Bracklesham, strongly resembles our species, in 

 having its margin free from the slightest appearance of crenulations, but it differs in 

 being rather less transverse and more tumid, with the posterior ventral margin less 

 angular and pointed, approaching in the last character the specimens from the 

 Coralline Crag ; these differences appear to be sufficient for specific distinction, and, 

 as yet, I have not seen any shell with which it can be fairly identified ; the exterior of 

 our Crag shell is smooth and glossy in specimens the surface of which has not been at 

 all eroded, and it was, in a recent condition, most probably covered with an epidermis. 

 On the anterior dorsal margin there is a rather narrow row of prominent angular teeth, 

 varying from 20 to 35 ; while on the short or posterior side the teeth are broader and 

 closer, and in number about 10 or 12, with a deeply impressed lunule on the posterior 

 or shorter side. There is no other exterior marking than the lines of growth, but 

 when the glossy surface is removed, the shell appears to have a radiated fibrous 

 structure, and the interior is often faintly rayed ; a long subangular depression for 

 the ligament curves inwardly towards the anterior, adhering to the inner edge of the 

 dorsal margin. The dorsal as well as the ventral margins are rounded, giving an ovate 

 form to the shell, only truncated on the posterior side, where the ventral margin forms 

 a sort of incipient rostrum, connecting it in that character with the following genus, 

 though less so than in some other species. 



2. Nucula Cobboldi^, /. Sowerby. Tab. X, fig. 9, a — b. 



Nucula Cobboldi^. J. Sow. Min. Conch, t. 180, fig. 2, 1818. 



— — Woodward. Geol. of Norf., p. 44, 1833. 



— — Lyell. Mag. Nat. Hist., New Series, vol. iii, p. 328, 1839. 

 — Id. Elem. Geol., p. 299, fig. 113, 2d. ed., 1841. 



— — S. Wood. Mag. Nat. Hist., New Series, vol. iv, p. 295, 1840. 



— — Id. Catalogue, 1840. 



— — G. B. Sowerby. Genera, No. 17, fig. 9. 



— — Forbes. Geol. Surv. Gr. Brit., p. 83, 1846. 



— — Morris. Cat. Brit. Foss., p. 94, 1843. 



Spec. Char. Testa transversa, oblique-ovatd, convexd, clausd ; irregulariter radiatd, 

 aut lineolis flexuosis ornatd ; intus scepe incrassatd ; margine integerrimo. 



Shell transverse, obliquely ovate, convex, thick and closed ; sculpture on the 

 exterior, with irregular radiations or lines in a divaricating or zig-zag direction ; 

 interior often irregularly thickened, margin smooth. 



Longitudinal diameter, \\ inch. Height, 1 inch. 



Locality. Red Crag, Sutton, Bawdsey, near Ipswich, and Felixstow. 

 Mam. Crag, Thorpe, Bramerton, Chillesford, Bridlington. 



