108 DIMYARIA. PETRICOLA. 
with or without lateral teeth ; internal margins crenated in 
some species ; ligament external. 
Egeria triangulata, Plate XI. fig. 7. 
The species of this genus are all fossil, and have only been met with in 
the Tertiary formation of Alabama, America. 
Genus XL.— GRATELOUPIA. — Moulins. 
Generic Character. — Shell equivalve, inequilateral, sub- 
cuneiform, anteriorly rounded, posteriorly subrostrated; 
hinge with three cardinal teeth, a series of five or six irre- 
gular, small divergent teeth behind the umbones, and one 
lateral, anterior tooth in each valve; ligament external ; 
muscular impressions two ; pallial impression situate pos- 
teriorly. 
Grateloupia Moulinsii. Plate XI. fig. 8. 
Known only in a fossil state. 
TRIBE II.— LITHOPHAGI. 
Boring shells, destitute of accessary pieces, and more or 
less gaping at their anterior side; ligament of the valves 
external. 
Genus XLI. — PETRICOLA. — Lamarck. 
Generic Character. — Shell equivalve, inequilateral, 
transverse, for the most part rather triangular, but some 
species are transversely elongated, and others subquadrate ; 
posterior side rounded; anterior side somewhat produced, 
more or less attenuated, and generally gaping; each valve 
provided with two cardinal teeth ; which in some instan- 
ces are curved and acute, especially the posterior tooth 
in the left valve, and the anterior tooth in the right ; the 
teeth are sometimes grooved internally, and the anterior 
tooth in one valve is broad and bifid ; and in some instan- 
ces the teeth are obtuse and short : two muscular impres- 
sions in each valve, that on the posterior side somewhat 
oblong, and the anterior one suborbicular; pallial impres- 
sion with a large sinus ; ligament external, but in some 
species nearly concealed by the prominent anterior margin 
of the valves near the beaks. 
Petricola laminosa. Plate IX. fig. 22. Distinguished 
from the Saxicava by having hinge teeth, and in being more 
regular in form. 
The Petricola inhabit the Ocean, and burrow in cavities of rocks, or wood. 
