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 attaims 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- 
eeous shell, straight, equivalve and regular, with a truncated, 
widely gaping posterior extremity ? Absolutely nothing ; 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 
