EXOGYRA. MONOMYARIA. © 79 
ing more distant and unequal by age. The lower valve is always the largest 
of the two, and generally the most concave ; and in the young condition they 
usually adhere to marine bodies. There are, in many species, a series of 
small denticulations situate near the hinge, which, however, cannot properly 
be considered as teeth. The ligament can only be reckoned as sub-external, 
although it is always concealed when the valves are closed; it is invariably 
placed in a subtriangular tripartite disk, one of its angles being always close to 
the umbo, from which diverge two somewhat elevated lines. 
Section 1. With simple margins, or slightly undulated; as in the 0. 
gigantea. 
Section 2. With the margins plicated, as in 0. carinata. 
There are several genera which may be confounded with this, as follow: 
Crenella and Perna have a series of longitudinal grooves; Pedum and Malleus 
are invariably attached by a byssus, while the Ostvea adhere by the external 
surface of their shells ; Lima and Pedum are always regularly formed bivalves, 
and attached by a byssus, the Vulsellce adhere by their hinge, and are usually 
found enveloped in pieces of sponge ; besides, the central portion of the hinge 
in the Vulsellz forms an internal projecting callosity. 
The Ostre@ are marine shells, and are met with in all climates. 
Fossil species are numerous, but difficult to make out. 
The Ostrea pulchra is found in the Plastic Clay, and 0. deltoidea, charac- 
terizes the Kimmeridge Clay. 
Mr Gerard, who traversed the snowy mountains of Thibet, in May, 1830, 
met with fossil shells at the amazing height of 16,000 feet above the level of 
the sea, and in reference to those of this genus he says, ‘‘ Just before crossing 
the boundary of Ludak into Brussalier, I was exceedingly gratified by the 
discovery of a bed of fossil oysters, clinging to the rocks as if they had been 
alive. In whatever point of view we are to consider the subject, it is sublime 
to think of millions of organic beings lying at such an extraordinary altitude, 
and of vast cliffs of rocks formed out of them, frowning over the illimitable 
and desolate waters, where the ocean once rolled.” 
Genus XXII. — EXOGYRA. — Say. 
Generic Character. — Shell inequivalve, and unequal 
sided, attached to extraneous bodies ; umbones spirally 
turned to one side; pit of the hinge curved and nearly 
linear ; the flat, free valve, provided with an obtuse tooth, 
which fits into a cavity parallel with the hinge pit in the 
convex attached valve; each valve furnished with one mus- 
cular impression. 
Exogyra conica. Plate VII. fig, 32, 33. Found at 
Blackdown, Parham Park, Chute Farm and Warminster. 
The shells of this genus are distinguished from those of Ostrea, which they 
somewhat resemble, by their impressed lateral spiral umbones, and the con- 
sequent linear form of the hinge pit, and by the parallel furrow in the attached 
valve, which receives the opposite striated tooth. It differs also from Gry- 
phza, being devoid of the lobe of that shell. 
