IGOCARDIA. “DIMY ARIA. 95 
position and manner of attachment of the ligament bring it near Cypricardia, 
but it is distinguished from the latter genus by its great thickness, depth, and 
oblique ridge which crosses the valves. 
Known only in a fossil condition, and is found in the lower portion of the 
Chalk at Lyme Regis. 
Genus XIII. — HIPPODIUM. — Conybeare. 
Generic Character. — Shell equivalve, obliquely trans- 
verse, very thick, deep, and inequilateral; umbones in- 
curved; ventral margin sinuated so as to produce a bilo- 
bate appearance ; hinge much thickened, and furnished 
with one rugged oblique tooth. 
Hippodium ponderosum. Plate VII. figs. 21 and 23. 
Found in a fossil state only. They occur in the upper beds of the Lias. 
Genus XIV. — HIPPAGUS. — Lea. 
Generic Character. — Shell equivaive, elongated, cordi- 
form, ventricose, with incurved beaks: destitute of cardinal 
teeth. 
Hippagus Isocardivides. Plate XI. fig. 5. 
Known only in a fossil state. 
Genus XV. — MEGALODON. — Sowerby. 
Generic Character. — Shell bivalve, equivalve, Iongi- 
tudinal, accuminated towards the beaks; a large bifid tooth 
placed upon a septum across the beak of the right valve, 
and one irregular and more acute tooth, similarly situate in 
the left valve; a small pit near the teeth for the reception 
of the ligament, which is anterior, long and external. 
Megalodon cuculatus. Plate VII. fig. 29. 
This genus is nearly allied to Mytilus, but is at once distinguished by the 
large teeth at the hinge, and its ponderous valves. It has also some relation 
to Myochoncha, but the only muscular impressions which are visible, from 
their situation, more nearly connect it with the Mytili. 
Known only in a fossil condition, and found in the Limestone at Bradley, 
near Newton Bushel, Devonshire. 
Genus XVI.— ISOCARDIA. — Lamarck. 
Generic Character. — Shell equivalve, heart-shaped, ven- 
tricose ; beaks very distant, divergent, and involute ; hinge 
with two primary compressed teeth in each valve, the one 
next the apex inflected under the umbo; and with one 
elongated, lateral tooth, situate immediately before the liga- 
ment, which is external, and divided into two ligaments at 
its posterior extremity, both of which are divergent to the 
