﻿SUPPLEMENT TO THE BIVALVIA. 



17 



Length, 1 inch ; height, f ths of an inch. 

 Locality. Headon Hill (S. Wood). 



Two specimens of what I have here considered a new species were found by myself at 

 the above locality fifty years ago, and I am unable to refer them to any particular division 

 of the beds that make up Headon Hill. Unfortunately, also, both of them have the hinge 

 not quite perfect, but they undoubtedly belong to the present genus. The exterior and 

 outline of my shell appear to come very near to the figure of Cyrena angustidens, Melle- 

 ville, given by Deshayes in ' An. sans Vert, du Bas. de Par.,' t. i, pi. 37, figs. 1, 2 (p. 515). 

 My shell, however, seems from M. Deshayes' figures to differ from angustidens in being- 

 thinner. It is also less elongated than the figure given of that same species by M. Melleville, 

 who speaks of it as " epaisse, assez profonde et tres oblique." So far as its imperfect state 

 allows of an opinion, also, the hinge of the British fossil seems to be narrower. Under these 

 circumstances, and as the French species is described as occurring only in the " Sables 

 Tert. inferieurs," while mine comes probably from a higher horizon, I have thought it 

 better to give my shell provisionally under a separate name, though strongly suspecting 

 its identity with the French species above mentioned. 



Cyrena transversa, Forbes, is figured in PI. Ill of the ' Geological Survey Memoir 

 on the Isle of Wight,' described at p. 149 of the same memoir in the following 

 words : 



" Testa transversa, depressa, angusta, insequilaterali, laevi postice producta, truncate 

 antice attenuata, rotundata, margine ventrali leviter arcuato." 



The interior of the shell is, however, not represented, nor is the dental furniture 

 described ; and judging from the figure given, it appears to me doubtful if it belongs to 

 the present genus. 



CYCLAS, Bruguiere, 1792. 



SpHjErium. Scopoli, 1777. 



Generic Character. Shell equivalve, subequilateral, more or less ventricose, thin, and 

 closed ; smooth or slightly marked by lines of growth. Hinge with a single cardinal 

 tooth in one and two in the other, and a distant lateral tooth in each valve. Impressions 

 of the adductors shallow or indistinct. Palleal impression small ; connexus ligamental 

 or external. 



Priority of name (as stated in ' Crag Moll.,' vol. ii, p. 106) properly belongs to 

 Sphcerium of Scopoli, but Cyclas has been so long in use and has been so generally 



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