82 MONOMYARIA. PECTEN. 
in a geological point of view ; as different species characterize the various strata 
of limestone, from the carboniferous limestone-up to the chalk. The Oolitic 
beds which intervene betwixt these abound with Plagiostome. P. gigan- 
teum is abundant in the Lias, and the P. spinosum is met with in great pro- 
fusion in the upper chalk. 
Genus XXVII.— DIANCHORA, — Sowerby. 
Generic Character Shell inequivalve, subtriangular, 
oblique, adherent ; attached valve provided with an angular 
hiatus, instead of an umbo; the other valve auriculated, and 
with an obtuse umbo; hinge destitute of teeth. 
Dianchora striata. Plate XI. figs. 1, 3. 
The Dianchore strongly resemble the Plagiostomata, but differ in being 
always attached, and in being provided with an opening instead of an umbo 
in the attached valve. The genus is known only in a fossil state, and occurs 
in the Greensand. 
Genus XXVIII. — PECTEN. — Bruguiere. 
Generic Character. — Shell inequivalve, the under valve 
generally more convex than the upper, subequilateral, with 
many grooves or ribs radiating from the umbones to the 
margins; provided with two ears, which are usually un- 
equal in size; close below one of them in the upper valve 
is a small notch for the passage of a byssus; muscular im- 
pression large, placed somewhat to one side; pallial im- 
pression destitute of a sinus; hinge linear, without teeth ; 
ligament consisting of three portions, of which the two 
lateral parts are elongated, and follow the hinge line, the 
third portion thick, triangular, and fitted into a central, 
triangular, shallow pit, within the hinge. 
Pecten cornea, Plate VII. fig. 11; and P. Dabdissont. 
Plate VII. fig. 6. 
This genus, consisting of numerous species of marine shells, may be sepa- 
rated into the following sections :— ‘ 
Section 1. Both valves somewhat convex, and not uniting all round, as in 
Pecten pleuronectus. 
Section 2. One valve flat, the other deep and convex, as in P, Jacobeus. 
Section 3. Both valves convex, equal in size or nearly so, as seen in P. 
turgidus. 
Section 4. Both valves convex, but unequal in size, as in P. bifrons. 
Section 5. Irregular, and seemingly adherent by their outside, to other 
bodies, as P. Dabissoni. Plate VII. fig. 6. The Baron de Ferussae formed a 
genus from this section under the name of Hinnites, but we do not consider 
there are sufficient distinctive characters to warrant the adoption of the 
genus. 4 
The Pectens inhabit the seas of almost all countries, and are remarkable for 
