SUPPLEMENT TO THE BIVALVIA. 17 
Length, 1 inch; height, $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 wp 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, pl. 37, figs. 1, 2 (p. 515). 
My shell, however, seems from M. Deshayes’ figures to differ from angustidens in beng 
thinner. It is also less elongated than the figure given of that same species by M. Melleville, 
who speaks of it as “‘ épaisse, assez profonde et trés 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. inférieurs,” 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 Pl. II of the ‘Geological Survey Memoir 
on the Isle of Wight,’ described at p. 149 of the same memoir in the following 
words : 
«esti transversi, depressi, angusta, ineequilaterali, laevi posticé producta, truncata, 
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, Braguiere, 1792. 
Spumrium. 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 
Spherium of Scopoli, but Cyclas has been so long in use and has been so generally 
c 
