10 
APPENDIX. 
IIERCODON. Conrad. 
Shell equivalved, hinge of left valve with two small cardinal teeth, an- 
terior one bifid, posterior one linear ; right valve with two diverging 
teeth, anterior one angular and excavated or angulated below the middle, 
long and very oblicpie ; cartilage pit deeply impressed ; anterior cardinal 
plate slightly furrowed longitudinally ; pallial line with a deep, regularly 
rounded, broad sinus. 
H. ellijpticus , pi. 2, fig. 2, 8, Shell elliptical, inequilateral, convex, 
disk entire ; beaks remote from anterior end which is acutely rounded, 
posterior end rounded ; umbonal slope undefined ; post-umbonal slope 
convex ; ventral margin regularly curved. 
The smaller outline figure represents what I suppose to be a young 
shell of this species, and is very abundant, while the large specimens are 
very rare. The largest found measures 3-|- inches in length. 
CYMELLA. Meek. 
Shell equivalved, thin in substance, gaping posteriorly ; hinge of right 
valve with a thin transverse tooth parallel to the shell’s length and approx- 
imate to, and below the cardinal margin, flat on the upper side and a 
curved pit above it ; fulcrum prominent, acute, straight ; left valve with a 
minute cardinal tooth anterior to the apex, approximate to the cardinal 
margin, and a much larger tooth under the apex, with a pit above it ; 
teeth all directed the same way. 
C. bella , pi. 2, fig. 9. Shell ovate, inequilateral, profoundly ventricose; 
substance extremely thin ; umbo broad and prominent ; disk sculptured 
with concentric furrows and acute radiating ribs interrupted at the fur- 
rows, anterior side nearly or quite destitute of radii and occasionally ex- 
hibiting minute raised points ; basal and posterior margins rounded. 
This genus appears to be very distinct from Pokomya, Forbes, although 
the external outline is very similar. The species is very beautiful and 
pearly, as thin as paper, and therefore I found only one entire small valve. 
The hinge of Plxoladomya irfrequens , as figured by Zittel, has a close 
resemblance to that of Cymella the principal difference being the absence 
of the anterior cardinal tooth which is a prominent character in Cymella, 
CORBEL A. Lam. 
In a small group of Cretaceous Coruulje the beaks of both valves arc 
spiral as in Mr. Gabb’s genus Paciiydon, but they differ from all Eocene 
