BIVALVIA. 23 



slightest mark of attachment. When it adheres to a cylindrical stem the shell is much 

 elevated, and the form of the body is communicated to the upper valve, giving it a ridge, 

 like that found in specimens of 0. dorsata. In thick individuals the ligamental area of the 

 lower valve is much elongated, while in thinner shells this space is very short. The upper 

 valve is plain, never radiated, but has merely concentric lines of growth : sometimes 

 it is slightly convex, while at others it is irregularly concave or with a depression in 

 the centre of the shell ; it is also sometimes strongly denticulated on each side of the 

 hinge-area; in one specimen I counted as many as twenty, in others there are scarcely 

 any. The lower valve also possesses these denticulations more or less, but their number is 

 no guide in specific determination. 



A fossil from the Nummulitic Formation of Cutch has been figured and described under 

 this name in the 'Geol. Tr.,' vol. v, pi. xxv, fig. 18, specimens of which arc in the 

 Museum of the Geological Society. 



O. angulaia, fig. 17, on the same plate, comes even nearer in form to jlabellida ; it is 

 possible that they may both be the same as the British species, which was very variable, 

 and had an extensive range. The specimen, Tab. VIII, fig. 5, a, b, is from Clarendon, and 

 although it presents differences, I have regarded it as a dwarf variety (viodicella), from 

 its general resemblance to the typical form. In this shell there is very great inequality 

 in the two valves. 



A specimen of the lower valve of this species has been met with in the Red Crag, at 

 Sutton, and in good condition. It is therefore probable that the bed which originally con- 

 tained it, and out of which it was derived, was not very remote from where it was found. 



This species is so generally known as a British fossil under the above Lamarckian name, 

 that I do not feel disposed to change it, although it ought to be called by Solander's 

 name, plicata. 



10. OSTREA GIGANTEA, J. Sowerbl/. PI. II. 



Ostrea gigantea. J. Sow. Min. Conch., t. 64, 1814. 



— — 1 Rousseau. Voy. dans la Russie Meridionale, vol. ii, pi. 4, fig. 1. 



— — ? Leymerie. Mem. de la Soc. Geol. de Fr., 2d scr., vol. i, pi. 17, fig. 2. 



— — 1 Baihj. Foss. Inv., from the Crim. Quart. Jour. Geol. Soc, vol. xiv, 



p. 143, 1858. 



— latissima. Desk. Coq. Foss. des Env. de Par., t. i, p. 336, pi. 52 and 53. 



— — Galeotti. Mem. Cour. par l'Acad. de Brux., t. xii, p. 151, pi. 4, fig. 18, 



1837. 



— — 1 De Verneuil and Desk. Mem. sur la Crimce, Mem. de la Soc. Geol. de 



Fr., t. iii, pt. 1, p. 19, pi. 4, figs. 1—3, 1838. 



— Pyrenaica? D'Orbigny. Prod, de Paleeont., t. ii, p. 317, No. 548, 1850. 



— gigantica. Solander, in Brand., Foss. Hanton., p. 36, No. 88, pi. 8, fig. 88, 1776. 

 _ _ Nyst. Coq. Foss. de Belg., p. 314, pi. 27, fig. lb; pi. 28, fig. \u, 



1843. 



— — Desk. An. sans Vert, du Bassin de Par., t. i, p. 108, 1860. 



