$0 DIMYARIA. ANODON 
some resemblance to a Chama, it is provided with a separate bony appendage ; 
the hinge, and its elongated muscular impressions will serve farther to distin- 
guish it. 
There ate numerous fossil Chamez, which are found in the Calcaire-grossier, 
Greensand, and London Clay. 
Genus I1. — DICERAS. — Lamarck. 
Generic Character.— Shell inequivalve, inequilateral, 
the one valve larger than the other ; attached to extraneous 
substances by the point of the beak of the larger valve, 
which is provided with one very large, concave, somewhat 
auriform, and thick cardinal tooth ; each valve furnished 
with two lateral, remote, muscular impressions ; umbones 
large, very prominent, divaricated, and somewhat irregularly 
and spirally twisted. 
The united valves of this shell present the appearance of 
Ram’s horns. 
Diceras acutus. Plate VIII. fig. 2, D. Lonsdali. 
Plate XI. fig. I. 1. 
Only two species of this genus are known; they are fossils; the former 
found in the Granular Limestone of Normandy, and in the neighbourhood of 
Geneva ; the latter in the Greensand of North Wiltshire. 
GRAND-DIVISION II. — LAMELLIPEDES. 
The foot of the animal depressed, lamelliform, and not 
posterior. 
TRIBE I.— NAYADES. 
Shells inhabiting fresh waters, — Hinge sometimes pro- 
vided with an irregular, simple, or divided tooth, and a 
longitudinal prolonged one ; sometimes toothless ; some have 
trregular, granulated tubercles, extending the whole length 
of the hinge line; provided with a compound muscular im- 
pression ; the umbones frequently decorticated. 
Genus III. — ANODON. — Bruguiere. 
Generic Character. — Shell equivalve, inequilateral, and 
transverse, for the most part very thin; hinge line nearly 
straight ; destitute of cardinal teeth; the hinge being 
glabrous with smooth lamina, truncated, or forming a sinus 
at the anterior end, terminating the apex of the shell; two 
lateral remote muscular impressions, the posterior one being 
compound ; muscular impressions of the mantle entire, and 
—- /~ . oo, 
