1 I Mr. J. Lycett on Trichites, 



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- 

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

 ; 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 ; 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 

 the thin and delicate Pinna is produced by a double structure, 

 by its substance consisting of two layers, the outer being fibrous' 

 tin; fibres placed perpendicular to the surface as in Trichites] 

 but the inner one is nacreous or lamellar, a contrivance which 

 effectually obviates the fragility which pertains to the fibrous 

 structure. Trichites on the contrary has one structure through- 

 out; the perpendicular fibres are crossed by a few extremely fine 

 parallel Unimr, which do not break off the continuity of the 

 . and impede fracture only to a very limited extent. The 



