140 MOLLUSCA FROM THE CRAG. 



Diameter, l^ths of an inch. 



Locality. Cor. Crag, Sutton, Rarasholt, Sudbourn, and Gedgrave. 

 Red Crag, Passim. 

 ]\Iam. Crag, Postwick and Thorpe. 



Recent, Mediterranean, Britain, Scandinavia, and North America. 

 This is one of our most abundant shells in the Coralline as well as in the 

 Red Crag. 



In the young state, it is nithcr less equilateral than in the adult, and has 

 comparatively a larger and deeper lunule, with the teeth more distinct and prominent. 

 It is nearly orbicular, though sometimes there is a trifling difference in the dimensions, 

 the diameter from the anterior to the posterior side exceeding that from the umbo to 

 the ventral margin, and vice versa. In the right valve are two cardinal teeth, one 

 large and l)ifid, the other small and simple, with a prominent lateral tooth on the 

 anterior side, and in the left arc also two cardinal teeth, one simple and the other 

 bifid, but their positions are reversed, the bifid one in the left being the anterior, with 

 a corresponding lateral tooth on that side. The interior is often strongly marked with 

 radiating striae, most distinct beyond the edge of the mantle mark : there are two deep 

 impressions by the adductor muscles, the posterior of an ovate form, the anterior one is 

 much elongated, being as it were bipartite, that nearest the anterior lateral tooth of 

 the usual form, with a ligulate prolongation down to near the middle of the shell ; in 

 addition to which, in the interior about the centre is a banded impression extending 

 half way across tlie shell, in a direction at right angles to the ligamental fulcrum, as 

 if the mantle had there a division. In some specimens, the exterior is covered witli 

 close-set lamellated striae, and the shell is somewhat tumid, while in others the shell is 

 flat or much compressed, and the concentric markings distant. A little inflection is 

 always to be seen on the posterior side, producing a more or less distinctly marked 

 sinus, and both sides are generally a little elevated, giving the shell the appearance of 

 being high-shouldered. A small but distinct lunule is visible, curving strongly near 

 the umbo, which gives to the young shell a comparatively larger mark there than 

 when it is full grown. This shell is said in the recent state to have a range from 

 low water-mark to the depth of 90 fathoms. 



2. LuciXA CRENULATA, S. frood. Tab. XII, fig. 7 a, b. 

 Llcina CKENULATa. S. Wood. Catalogue, 18-10. 



— STRIATUI/A? iVy«<. Recli. Coq. Foss. ile lloesselt et Kl. Sp., p. .">, No. II, pi. I, 



fig. 11, 1836. 

 — ? Nyst. Coq. Foss. Belg., p. 131, pi. 6, fig. 9, a—c, 18-l-J. 



— DEXTATA? Goldf. Pet. Germ., vol. ii, p. 230, t. N7, fig. 1. 



— CRENULATA. Coiirad. AiiKT. Mioc. Foss., p. 39, pi. 'I'd, fig. '1. 



Spec. Char. Testa purvd orbiculari, aqiiilaterali, convexu, concentrice striata, striis 

 numerosis confertis, lunula, iinpressd, clonffato-ovatd ; dentibm lateralibm distinctis; marffine 

 crennlatd. 



