212 MOLLUSCA 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 imBricata, J. Sowerby. Tab. XIX, fig. 3 a—/. 
ASTARTE IMBRICATA. J. Sow. Min, Conch., t. 521, fig. 1, 1826. 
_ — Nyst. Coq. Foss, de Belg., p. 153, pl. ix, fig. 3a, 6, 1844. 
Dosina rmBricaTa. S. Waod. Catalogue, 1840. 
Spec. Char. Testi suborbiculari, subcordatd, compressiusculd aut tumidd, crassd, 
imbricata, imbricibus crassis 12—16, reflexis, striatis ; natibus recurvis ; lunuldé ovato- 
cordatd ; marginibus crenulatis ; sinu palliari parvo, angulato. 
Shell suborbicular, slightly heartshaped, somewhat compressed, occasionally 
tumid, thick and imbricated, with 12—16 broad, thick, reflexed ridges or imbrications; 
umbones recurved; lunule impressed, elongate; margins crenulated; palleal sinus 
small and angular. 
Diameter, 1% inch. 
Localities. Cor. Crag, Ramsholt, and Gedgrave. 
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,) which, 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 lamellated 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 /. casina, differing only in the exterior ornament. 
Var. , or gibberosa, (fig. e,f,) 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 sharp, erect, and distant lamellz, 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 lamella being so close 
as to represent little more than lines of growth with 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 lamelle. 
