a fossil genus of Bivalve Mollusks. 345 



the horizon of science as an obscure and problematical object, 

 which it will be our present endeavour to bring more nearly 

 within the grasp of the conchologist. 



Generic Character. 



Shell of fibrous structure, thick, ovate, oblong, inequivalve, ine- 

 quilateral and irregular ; umbones terminal, produced and 

 funnel-shaped, the apex gaping. Hinge-margin oblique and 

 elongated, the margins undulated, anterior margin corrugated 

 and thickened beneath the umbones. Hinge lateral, linear and 

 without teeth. 



Our specimens, though not numerous, exemplify the genus in 

 a satisfactory manner, and place its characters free from ambi- 

 guity. The valves are both separated and in apposition ; in one 

 instance the interior of the cardinal border and terminal extre- 

 mity have been cleared, but the muscular impressions have not 

 been seen. The general figure is oblique and nearly quadri- 

 lateral, one valve being convex, the other flattened or even a little 

 concave ; the margins undulate, are rather irregular, including 

 even the hinge-line, and there is always a considerable undula- 

 tion occupying the posterior border, at which part the valves are 

 thinner, more expanded and flattened ; the undulations of both 

 valves correspond ; they are rounded, having no posterior trun- 

 cation, and when closed leave no hiatus. In the concavity of the 

 anterior border is a corrugation which marks the probable place 

 of exit for a byssus, a feature exactly corresponding with that in 

 Perna, Avicula, &c, but there is scarcely any distinct hiatus per- 

 ceptible. The umbones form a hollow funnel-shaped cavity nar- 

 rowing to the extremity, but open, the opening being rounded, 

 and formed by the termination of both the valves; the shell 

 about its middle and anterior parts attains a thickness exceeding 

 any recent bivalve, and comparable only with the fossil genus 

 Catillus ; the smaller valve is the thinnest. With respect to its 

 affinities, that to Pinna, which has engaged the attention of natu- 

 ralists, would appear, to say the least of it, to be very remote. 

 What do we find in conformity with a delicate, almost papyra- 

 ceous shell, straight, equivalve and regular, with a truncated, 

 widely gaping posterior extremity ? Absolutely nothing j on the 

 contrary we have a shell of monstrous thickness, very oblique, 

 inequivalve and irregular, its posterior side being neither trun- 

 cated nor gaping — in fact nearly every generic feature of import- 

 ance is reversed ; the fibrous structure common to both seems to 

 have misled observers into a supposed generic identity. But even 

 the structure of the two genera when carefully examined presents 

 a difference equally marked and characteristic : the strength of 



