CHAMA. DIMYARIA. 89 
Fossil shells of this genus are plentiful, especially in the Great Oolite, 
Pisolite, and Coral Rag ; they also occur in the marine Tertiary beds, and have 
by several authors been mistaken for Fistulane, owing to the decomposition © 
of the Madrepora which they inhabited, surrounding them, and producing the 
appearance of their being encased in an echinated sheath, 
ORDER ITI.— DIMYARIA. 
Shells with two distinct, remote, muscular impressions ; 
which are widely separated, and inserted towards the lateral 
extremities of the valves. 
GRAND-DIVISION I. 
Shells irregular, and always inequivalve. 
TRIBE I.— CHAMACEA. 
Shell inequivalve, irregular, attached to other bodies ; 
hinge with one or more large teeth, and provided with two 
separate lateral muscular impressions. 
Genus I. — CHAMA. — Bruguieére. 
Generic Character. — Shell irregular, thick, usually very 
inequivalve, for the most part covered with irregular spines, 
or foliated processes ; umbones distorted, unequal, distant, 
and involute ; that of the attached valve salient at the base, 
and in some instances projecting considerably beyond it, 
the other is for the most part reflected over upon its valve, 
appearing as if imbedded in it ; hinge with one strong, thick, 
irregular, oblique, striated, and generally crenated tooth in 
one valve, which fits into an irregular striated groove in 
the opposite valve ; each valve provided with two distant, 
lateral, muscular impressions ; line of the mantle attachment 
entire ; ligament external, subdivided at its posterior extre- 
mity ; one of the segments decurrent to the point of the 
umbo in each valve. ; 
Chama Haliotoidea. Plate VII. fig. 4. C. Ponderosa. 
Plate IX. figs. 27, 28. 
The Chame# are distinguished from the shells of the genus Diceras, by the 
latter having large, conical, divergent, spiral horn-shaped umbones, and in 
its larger valve having a large, subauriculate, concave, and prominent tooth; 
Isocardia is more regular in form; Spondylus will be distinguished by its 
triangular area between the umbones; and although the Cleidetheras has 
