96 DIMYARIA. CARDITA. 
point of the beak in each valve; both valves provided with 
two lateral, remote, muscular impressions, the linear im- 
pression of the mantle is entire, and extending from one 
muscular impression to the other. 
Isocardia minima, Plate VIII. fig. 24, 
The Isocardie are distinguished from the shells of the genera Chama and 
Diceras, by being equivalve, and in the ligament being dichotomous, and ex- 
tending to the point of the umbo. 
The Isocardiza are Oceanic shells, and the recent species are few. Fossil 
species occur in the Kelloways rock, the Crag and London Clay: and in a 
yellow coloured Limestone at Coutances in Franee. 
Genus XVII. — CYPRICARDIA, — Lamarck. 
Generic Character. — Shell equivalve, inequilateral, sub- 
quadrate, obliquely or transversely elongated ; posterior side 
very short; hinge with three teeth in each valve, situate 
immediately within and behind the umbo ; and one rather 
lengthened lateral tooth extending towards the anterior 
side; each valve with two somewhat irregular, lateral mus- 
cular impressions ; mantle or pallial impression very indis- 
tinct and nearly obsolete ; ligament external. 
Cypricardia carinata, Plate X. fig. 5. 
The Cypricardie inhabit the ocean, and are distinguished from the Cardite, 
which they resemble very nearly in form, by having three cardinal teeth situate 
under the beaks. It is supposed that they affix themselves to marine bodies 
by a byssus. 
Fossil species occur in the Blue Maris, south of France, and in the Silurian 
rocks of Britain. 
Genus XVIIL — CARDITA. — Lamarck. 
Generic Character. — Shell equivalve, inequilateral, sub- 
orbicular, subquadrate, transversely oblong, in some in- 
stances; more or less grooved exteriorly, emanating from 
the umbo, and terminating in the margin: lips crenulated 
interiorly ; two generally oblique teeth in the left valve, one 
of which is elongated, thick, and for the most part curved ; 
and in the right valve one elongated, thick, and oblique 
tooth, with a deep, lengthened cavity, for the reception of 
the large tooth of the opposite valve; in some species 
there is an additional indistinct tooth in this valve ; two 
somewhat ovate lateral muscular impressions in each valve ; 
mantle muscular impression entire; ligament external. 
Cardita lunulata. Plate VIII. figs 26, 27. 
Distinguished from the Gypricardize by the number of the cardinal teeth ; 
and although closely allied to Cardium, the shells or that genus are nearly 
