84 MONOMYARIA. CRENATULA. 
Sowerby has united this genus with Meleagrina, and remarks that the general 
form of these shells is somewhat square, with their superior angles rounded. 
The Meleagrine approach nearer the circular form than the Avicule, which 
isa little more transverse. In the latter genus, the two valves are rather more 
conspicuously unequal than in Meleagrina, which is distinctly marked by 
their pearlaceous interior. The line of the hinge in Avicule is transverse 
and straight, but in some species its extremities are very short as in Melea- 
grina, but extremely variable in length, and sometimes excessively prolonged. 
The left hand valve is contracted and notched at the posterior side near the 
base ; which is less observable in the right hand valve ; this is destined for the 
passage of the byssus. Each valve is generally provided with a single small 
tooth, placed immediately within the beaks; and is frequently met with in 
the Meleagrinz, contrary to the assertion of Lamarck. The area of the liga- 
ment is marginal, linear, narrow, and dilated in the centre ; which is very con- 
spicuous in old shells, forming an obliquely triangular pit, emanating from 
below the beaks, and increasing gradually in width towards the centre of the 
valves. This is common to both Avicula and Meleagrina, The external 
rows of imbricated scales are also common to both genera ; the muscular im- 
pression large, suborbicular and in both nearly central, with a row of minute 
ones running from the inner edge to the umbo. 
The shells of this genus are oceanic, principally inhabiting the seas of tro- 
pical climates ; a few species are found in Europe, and two are known to 
exist in the British Seas. 
Fossil species occur in the Supercretaceous rocks of Dax and Bordeaux, also 
in the Cretaceous series, and are rather plentifulsin the Oolitic group. 
Genus XXXII. — GERVILLIA. — Defrance, 
Generic Character. — Shell oblong, nearly equivalve, 
greatly inequilateral and oblique; hinge line rather long, 
linear and nearly straight, with numerous irregular, some- 
what transverse, small pits, and teeth situate below the dorsal 
edge. 
Gervillia aviculoides. Plate VII. figs, 30, 31. Found 
in the Shanklin sand, Shotover Hill, Oxford, and in the 
Greensand near Lyme Regis. | 
It is not known whether this shell is furnished with a byssus, and the liga- 
ment cannot be described. It resembles an Avicula in general form, and the 
hinge approaches to that of a Perna. 
Known only in a fossil state ; and from the shells with which it is found asso- 
ciated, is supposed to be a marine species, They are met with in all the strata 
from the Lias to the Baculite Limestone. 
Genus XXXII — CRENATULA. — Lamarck. 
Generic Character. — Shell sub-equivalve, flattened, 
somewhat distorted and lamellar ; hinge lateral, linear, mar- 
ginal, and internally crenulated ; the crenulz formed in a 
continuous series along the hinge, each of them presenting 
a small rounded ecallosity, and excavated for the reception 
