404 THE MEDALS OF CREATION, Cuap. XI. 
tricts; and their origin would be difficult to understand 
without this explanation.* 
Avicuta. Lyell, p. 274.—Above fifty species of this 
genus of shells have been found in the British strata; their 
general character will be readily understood by reference to 
the pearl-oyster, (A vicula margaritifera,) which is so largely 
imported for the manufacture of mother-of-pearl ornaments. 
A remarkable species is found in the Lias, called, from the 
great disproportion in the size of the shells, Avicula inequi- 
valvis, (Lyell, p. 274.) The recent species are inhabitants of 
warm Climates. 
Our limits will not admit of further notice of the M/ono- 
myaria, and we proceed to the second division of the 
plated-gilled mollusca. 
Dimyaria: Bivalve Shells, with two muscular imprints. 
The conchifera, or bivalve shells, of this group, found 
fossil, are more than double in number those of the prece- 
ding; nearly eight hundred species are known in the rocks 
of Great Britain, of which by far the greater number is 
marine. But we must restrict our notice of this division 
to a few génera, that more space may be devoted to that 
important class, the Cephalopodous Mollusca. 
The Cardium, Venus, and Mussel shells, are familiar 
examples of the Dimyaria, The conglomerates, now form- 
ing in the British Channel, from accumulations of the 
recent species of Cockle (C. edule), have been previously 
noticed; see Lign. 124, p. 386. In the strata of England 
* The Rev. W. Conybeare first ascertained the origin of these 
fossils, and figured and described them in an elegant Memoir, pub- 
lished in Geol. Trans. vol. ii. first series. Mr. Morris proposes the 
name of Clionites for the fossil bodies derived from the depredations 
of the Cliona on the Inocerami and other shells. See Annals Nat. 
Hist. 1851, and my Pictorial Atlas of Organic Remains. 
