78 MONOMYARIA. OSTREA. 
and Terebratula by the structure of its hinge and very peculiar muscular im- 
pressions, and neither of these genera have a perforation near the hinge, as 
in the Anomiz. 
The species are limited in number, and inhabit the ocean ; they are met with 
in almost all climates. 
Anomiz are rare in a fossil state, and occur in the marine formations above 
the chalk in France, and the Crag and London Clay of England. 
SUB-DIVISION II. — Ligament semi-interior ; shell foliaceous, frequently 
very thick. ‘ 
Genus XX. — VULSELLA. — Lamarck. 
Generic Character. — Shell longitudinal, equivalve, sub- 
irregular; beaks equal, somewhat separated ; between these 
lies the useless and decayed remains of the ligament ; valves 
united at the hinge by a semi-internal ligament, affixed in 
each valve to a subtriangular tripartite disk, one of its points. 
reaching close to the beak, and a little inclined to one side ; 
its central portion hollow, and forming a projecting callosity 
within the shell, with the chief part of the ligament attached 
to this conical hollow; each valve is provided with a single 
lateral muscular impression, which is oblong and contracted 
towards the base. 
Vulsella deperdita. Pilate 1X. figs. 80, 31, 32. Found 
fossil in the Calcaire-grossier at Grignon. 
The Vulselle differ from the Crenatule chiefly in the hinge being provided 
with only one callosity, while there are many in Crenatula. ~ 
Sometimes the species of this genus gape posteriorly. They are but few, and 
inhabit the ocean, chiefly the Indian seas; and one is found in New Holland. 
Genus XXI.— OSTREA. — Linneus. 
Generic Character. — Shell inequivalve, irregular, and 
foliaceous ; umbones somewhat separated, and of unequal 
size ; lower valve largest, concave and adherent; upper 
valve smallest, and somewhat plain; hinge destitute of 
teeth ; ligament partly external; the facet to which it is 
attached is subtrigonal and tripartite, and is divided by two 
elevated lines which divaricate from the umbo; each valve 
provided with two muscular impressions, the one large, 
suborbicular, and nearly central; the other very small and 
situate near the hinge. 
Ostrea gigantea. Plate VII. fig. 14. 
All the shells of the genus Ostrea are of a laminated foliaceous structure. 
These lamina are composed of perpendicular fibres ; the points of the umbones 
are always apart from each other, and in some instances very remote, becom- 
