1G6 MOLLUSCA FROM THE CRAG. 



the fossil from Angers is, in all probability, the same, as I have no doubt the one figured 

 and described by M. Nyst (C. squamtdosa) is no more than a variety of this Protean 

 shell. The valves are always- very thick and strong, ornamented with rounded 

 radiating ribs ; these are covered with more or less rugose lines of growth at all 

 times, and in some wcll-prcscrvcd specimens large and elevated squamae may be 

 observed at somewhat irregular distances ; the spaces between the ribs, upon the 

 centre and anterior part of the shell, are rather narrower than the ribs themselves, 

 more especially on the posterior part, where they are also less elevated. The umbones 

 are slightly involute, and have a small plain space or lunule immediately beneath them. 

 The right valve has one large tooth sloping towards the posterior side, with the rudi- 

 ments of a small one before it, near the umbo ; in the left valve is a large depression 

 for the tooth of the oj)posite one, and a thin, linear, compressed tooth, nearly parallel 

 to the dorsal margin, with a small triangularly-formed cardinal tooth, that becomes 

 obsolete in some specimens : these elongated teeth are marked with vertical striae like 

 the lateral teeth in some species of Mactra, CyrencB, &c. 



In those species with a very transverse form of valve, the shell is generally more 

 compressed, and I'ke versa, in the orbicular varieties it is more tumid ; deep impressions 

 are left by the adductors, and the margin of the shell is indented by the elevation of 

 the ribs ; a small additional muscle mark is left upon the shell joining the anterior 

 adductor, such as is common to most of the species of Astarte and some of the 

 Venerida;, and the ligament was probably strong and powerful, as indicated by the 

 fulcrum, and deeply impressed furrow on the outside. 



In all n>y specimens, notwithstanding its extreme range in variation in regard to 

 form, from that which is truly orbicular and nearly equilateral, to those which are more 

 especially transverse and very inequilateral, they invariably possess the character of 

 being somewhat rounded at the posterior side, and never angulated, like Ch. antiquata, 

 Linn. I am not sufficiently acquainted with the Italian fossils to say whether either of 

 the shells figured by Brocchi could be united with the Crag shell, but those specimens 

 I have seen appear to differ in being also less rounded on the posterior side, with a 

 slight difference in the form of the ribs, and covered with more prominent scales. 



2 Cardita scalaris, Lealhes MSS. Tab. XV, fig. 5. 



Venekicakdia scalaris. J. Sowerby. Min. Conch., t. 1!)0, fig. .3, 1825. 



— — Nijst. Rech. Coq. Prov. d'Anv., p. 12, No. 47, 1835. 



— — Potie: et Mich. Catal. des Moll, dc Douai, p. I(!C, No. 19, 184 I. 

 Cardita — Gold/. Pet. Germ., vol. ii, p. 188, t. VM, fig. 2 a, b. 



— — Nyst. Coq. Foss. de Belg., p. 213, pi. xvi, fig. 9 a, b, d, 18-1-1. 



Spec. Char. Testa orhiculato-trianyulari, depressd ; costis 20 — 22 convcxis, nodosis ; 

 concentrice sulcatis ; umbonibus medianis. 



