i DW 
py ; LÀ * 
zx Y * r w AMET 
* e: Family Osteodesmacea. 
Osteodésma trapezoidàlis, Buarnv., Mal. p. 660, edie 
lineata, tab. 16, f. sub nom. ANATINA trapezoides. > 
Periploma, Rane, Man. des Mollus. p. 325. Spon, 
a genere exclus. 
' Periplóma humi died: die yc. Meth. Vers, 
> P- 73 
sH., Lam. Anim. sans Vert. edit. 1835, 
^ DESCRIPTION: Sheli very thin and fragile, trans- 
verse, sub-quadrangular, very inequivalve, corbuliform, 
right valve the larger and quite ventricose, left valve ` 
opercular and nearly flat ; extremely inequilateral, pos- : 
terior side niuch the icd both extremities sub-trun- 
cated, the anterior in its whole extent, the posterior in 
its lower portion ; anterior cardinal edge nearly recti- 
linear, the basal margin parallel to it, the posterior de- 
clining rather abruptly to the commencement of the 
truncation ; the hinge i$ without teeth, but each valve - 
is furnished with a r spoon-shaped process, pro- 
jecting obliquely forward, and forming, with the anterior 
cardinal edge, a profound sinus, which receives a thin, 
flattened, semicircular ossiculum, fitting in edgewise with 
the convex edge outermost, the inner edge a little 
concave, so that when the valves are opened, the ex- 
tremities of this appendage are in a manner hooked 
under each cardinal edge. The ligament is twofold, 
the internal one being very strong, and inserted into the 
cardinal processes, while the external is nearly mem- 
branous, and remarkable for being placed anteriorly, 
instead of posteriorly, as in almost all other shells. A 
carination, rather sharply bd. on the ram valve and 
VOL. IIH.— NO. 11. 
