70 MONOMYARIA. SPIRIFER. 
species it is so thin that it is hardly observable; yet this valve is the most cha- 
racteristic of the genus; it has the property of adhering to other substances by 
its own surface ; it is wholly destitute of a hinge, teeth, or ligament. It has 
four strong impressions by which the tendinous muscles are attached, and by 
which it is at once united to the animal, and the upper convex valve; two of 
these muscular impressions are situate at a distance from each other, near the 
margin, which is usually nearly straight between them, the others are towards 
the centre of the disk, merely separated from one another by a slight promi- 
nence in the under valve, while they are more remote in the upper valve; the 
inner surface, particularly about the margin, is granulated; produced from 
the structure being cellular, instead of foliaceous. The vertex of the upper 
valve is nearly central; it is generally thin in substance, and presents the 
same granulated surface near the margin, as the lower valve, although not 
quite so conspicuous. The cellular structure of the shells of the genus Crania, 
at once serve as an unvarying mark whereby to distinguish them from all other 
genera. They also differ from Hipponyz, in having four muscular impressions 
in place of two. 
The Craniz are marine shells, and very limited in number ; they inhabit the 
Mediterranean, and British seas. 
There are only five or six fossil species of this genus; and these are found in 
the Calcaire-grossier near Paris, and one at Népou, department of la Manche, 
another at Mendon; and one also in Sweden. 
Genus IV.— THECIDIUM. — Defrance. 
Generic Character.— Shell inequivalve, somewhat irregu- 
lar, and nearly equilateral ; umbones acute, imperforate ; 
attached by the outside of the convex valve; lower valve 
provided with two internal, short cardinal processes, and an 
external triangular area, rather irregular in its form, and ex- 
tending to the umbo ; upper valve somewhat flattened, witha 
small, short obtuse appendage, situate externally, at its base; 
furnished with two small. lateral, cardinal processes inter- 
nally, and with variously curved Jamine, invariably adhering 
to the inner disk of the valve; hinge destitute of a ligament, 
but the valves are kept together by the attachment of the 
lateral processes. 
Thecidium recurvirostrum. Plate IX. figs. 1 & 8. 
The shells of this genus have considerably the aspect of several of the Tere- 
bratulz, but may at once be distinguished by their being devoid of an umbonal 
perforation in the convex valve ; which is an invariable characteristic of that 
genus, 
This is a marine genus, and fossil species have been found in the Chalk of 
Orglandes, in Normandy, and at Maestricht. 
Genus V.—SPIRIFER. — Sowerby. 
Generic Character. — Shell transverse, equilateral, in- 
equivalve, hinge straight, linear, widely extended equally 
on both sides of the umbones, which are more or less 
