212 xMOLLUSCA FROM THE CRAG. 



served: in this species the ridges are more numerous about the umbo or in the young 

 shell, whereas in the other they are more prominent and distant, and in the mature 

 state this species never attains to so great a size : the posterior side is sometimes 

 slightly pointed, and the length is generally greater than the height, but occasionally 

 the greater dimensions are the reverse. In the living state it is said to have a vertical 

 range from four to sixty fathoms. 



3. Venus imbric.vta, /. Sower/ji/. Tab. XIX, fig. 5 a—/. 



AsTAUTE IMBIUCATA. J. Sow. Mill. CoDch., t. 521, fig. 1, IS'iG. 



— — Ni/st. Coq. Fobs, de Belg., p. 153, pi. ix, fig. :ia, b, IS 11. 



DosiNA IMBIUCATA. S. Wood. Catalogue, 1840. 



Spec. Char. Testa suborbiculari, subcordaid, compressimculd aid tumidd, crassd, 

 imbricatd, imbricibus erassis 12 — 16, rejlexis, striatis ; natibus recurvis ; lunula ovafd- 

 cordatd ; marginibus crenulatis ; sinu palliari parvo, angulato. 



Shell suborbicular, slightly heartshaped, somewhat compressed, occasionally 

 tumid, thick and imbricated, with 12 — 16 broad, thick, reflcxed ridges or imbrications; 

 umbones recurved ; lunule impressed, elongate ; margins crenulated ; palleal sinus 

 small and angular. 



Diameter, \\ inch. 



Localities. Cor. Crag, Ramsholt, and Gcdgrave. 



Red Crag, Sutton, Walton-on-the-Naze. 



This species is not particularly abundant either in the Coralline or in the Red Crag, 

 and appears in both to be subject to considerable variation. 



There are two very distinct forms in this species : what may be called var. a, 

 (fig. c, d,) Avhich, in its extreme form, is an elegant shell, curving considerably towards 

 the anterior, and ornamented with about sixteen thick, obtuse, but reflected ridges, at 

 nearly regular distances, the fine lamcllated ornament of the young shell is worn down 

 in those from the Red Crag, from which Formation only I have seen this variety. In 

 this state it approaches very near to F. casiiia, differing only in the exterior ornament. 

 Var. /3, ox gibberosa, (fig. c,/,) on the contrary, is very inelegant, being a thick, tumid, and 

 clumsy looking shell, with a hunchedback and, in most instances, bipartite, the older 

 portion having a very different appearance from that of the young shell, in which the 

 surface is furnished with a few sharj), erect, and distant lamellne, about eight or ten, 

 and these, at the extreme edge or posterior margin, terminate abruptly, and are sharp 

 and almost pointed, after which the shell is nearly smooth, the lamellae being so close 

 as to represent little more than lines of growth witli an irregular surface. In this 

 variety the shell is nearly orbicular, very tumid, and is rather higher than it is long, 

 the other variety is more compressed, and the diameter is greater in a longitudinal 

 direction ; between these extremes there is every imaginable variation, and the specimen 

 represented in ' Min. Conch.,' appears somewhat of an intermediate character, having 

 the surface flattened by the abrasion of the lamellae. 



