' 
88 MONOMYARIA. LITHODOMUS, 
The shells of the genera, Modiola and Lithodomus, may be mistaken for 
Mytili; but will be at once recognized by their umbones not being terminal as 
in the Mytili. 
The shells of this genus are all marine. They abound on all rocky shores, 
and in the estuaries of rivers. 
Some of the species are provided with minute dentieles within ~he point 
close below the beaks, which, however, have no connection with, or influence 
upon, the hinge. 
A few fossil species of the genus Mytilus are known, they are met with in 
the Crag, and occur in the strata both above and below the Chalk. 
Genus XXXVIII. — MODIOLA. — Lamarck. 
Generic Character. — Shell subtransverse, equivalve, 
regular, oblique ; form oblong, somewhat wedge-shaped, and 
greatly inequilateral ; anterior side very small and obtuse ; 
posterior side rounded and closed ; anterior margin slightly 
gaping for the passage of the byssus, and forming with the 
base a line oblique to the dorsal one ; beaks nearly lateral ; 
outside covered with a strong horny epidermis ; hinge 
without teeth ; ligament elongated and subexternal ; two 
muscular impressions, the posterior one large, sublateral, 
elongated, and irregular, the anterior one small and ter- 
minal ; the pallial impression irregular, and destitute of a 
sinus. 
Modiola cuneata. Plate VIII. fig. 22. 
The Modiole are distinguished from the Mytili, by their beaks not being 
terminal, but placed a little within the posterior or smaller side of the valves. 
The shells of this genus inhabit the ocean, and four species are found in the 
British Seas. They occur fossil in the Oolitic series, and also in other beds 
both above and below the Chalk. 
Genus XX XIX. — LIFHODOMUS. — Cuvier. 
Generic Character. — Shell transverse, equivalve, regu- 
lar, elongated, and of a cylindrical form when the valves 
are shut; rounded at both extremities; anterior side very 
short ; external surface covered with a strong horny epider- 
mis ; beaks placed anteriorly, and hardly prominent ; hinge 
destitute of teeth, linear, the hinge line forming a greater 
or less obtuse angle with the posterior margin ; ligament 
linear and internal, with a small portion exposed externally ; 
two indistinct muscular impressions, the anterior ane very 
small, the posterior one large and oblong. 
Lithodomus antiquus. Plate X. fig. 27. 
The Lithodomi are all sea shells, inhabiting the West Indies, Mediterranean, 
and all coral rocks, 
