AVICULA. MONOMYARIA. 83 
the beauty and diversity of their colours and markings; the valves, in most 
instances, being of different shades and hues. 
They abound in a fossil state from the Crag to the Oolitic series of strata. 
Genus XX1X. — LIMA. — Bruguiere. 
Generic Character. — Shell longitudinal, equivalve, ine- 
quilateral ; sides somewhat thickened, and gaping ; um- 
bones divergent, their internal facets inclined outwards ; 
hinge provided with two lateral teeth, one on each side in 
both valves, which become nearly obsolete in adult shells ; 
area between the beaks, to which the ligament is attached, is 
divided ; tripartite ; the middle or hinge pit is rounded above, 
and contains the chief portion of the ligament, the remaining 
portions are attached to the somewhat elongated, linear 
divisions ; muscular impression lateral, suborbicular, from 
the inner margin of which the muscular impression of the 
mantle emanates, and traversing the other side of the valves 
in a circuitous form, appears to terminate near the beak ; 
external surface covered with a very thin epidermis. | 
Lima gibbosa, Plate VII. fig. 1. 
The Lime are marine shells, and inhabit .the seas of almost all countries. 
They may be distinguished from the Ostrea by their regularity of form, and 
by never being attached externally. Their oblique form separate them from 
Pecten. The Lime are supposed to be attached to other bodies by a byssus. 
The shells of this genus abound in a fossil state; they are met with in the 
inferior Oolite, the Calcaire-grossier of France, and in the same strata of 
Italy, and the London Clay of England. They have not, however, been ob- 
served in any of the strata below the Lias. 
GRAND-DIVISION III. 
Shells with an elongated marginal ligament. 
‘TRIBE I.— MALLACEA. 
Shells foliaceous, more or less inequivalve, with the liga- 
ment marginal, partly linear, and either simple or inter- 
rupted by crenulations. 
Genus XXX.— AVICULA. — Lamarck. 
Generic Character. — Inequilateral, inequivalve, folia- 
ceous, sub-quadrate and oblique, pearlaceous within ; hinge 
rectilinear, and produced on each side into straight auriform 
appendages, with a small indistinct tooth in both valves, an 
elongated, marginal ligamentiferous area, widened near its 
centre. 
Avicula echinata. Plate VIII. fig. 15. Found in the 
Cornbrash Limestone of Wiltshire and Dorsetshire. 
