PACHYODON. DIMYARIA. 103 
sions in each valve, that of the mantle entire, and destitute 
of a sinus ; ligament external and slender, 
Cyclas deperdita. Plate VIII. fig. 1. C. levigata, 
Plate X. fig. 7. 
The Cyclades appear to be found only in the continents of Europe and 
America. They inhabit lakes, ponds, ditches, and slow running streams. 
Fossil species occur in the recent lacustrine formations, and the Blue Marls of 
France. 
GRAND-DIVISION Ill. — TENUIPEDES. 
The mantle barely united before ; foot small, narrow, and 
compressed ; shell having but a moderate gape. 
TRIBE I.— NYMPHACEA. 
Fiaving never more than two primary teeth in the same 
valve ; shell often gaping slightly at the lateral extremities ; 
ligament external ; umbones generally projecting outwards. 
SUBDIVISION I. — Destitute of lateral teeth. 
Genus XXXI.— CRASSINA. — Leach. 
Generic Character. — Shell suborbicular, transverse, 
equivalve, inequilateral ; hinge with two strong, divergent, 
cardinal teeth in the right valve, and two unequal primary 
teeth, and one small, nearly obsolete tooth, together with 
an indistinct lateral one in the left valve ; two ovate or 
oblong, remote, lateral, simple muscular impressions in each 
valve, with a third very small one, situate immediately 
below the indistinct lateral tooth, or at the end of the pos- 
terior external depression, and in some instances mingling 
with the lower termination of the posterior muscular im- 
pression, which is always simple and not sinuated ; ligament 
external. 
Crassina lurida. Plate VIII. fig. 8. 
The Crassine are marine shells, and do not appear to be met with in the 
tropics. One species inhabits the Northern Ocean. They are distinguished 
from Crassatella by their external ligament. 
Fossil species are numerous, and occur in the lower Oolite, the English 
Crag, and Greensand. 
Genus XXXII. — PACH YODON, — Stutchbury. 
Generic Character.—Shell transverse, elliptical, equi- 
valve, inequilateral, thick ; hinge very strong, with one 
oblique, thickened, cardinal tooth in the right valve, with a 
