302 Lamarck’s Genera of Shells. 
two muscular impressions, and from the chamacea, by the shell 
being regular and free. 
Type. Pandora rostrata*. (Tellina inequivalvis. Linn.) 
Anterior side of the shell longest; attenuated, beaked, and 
angular, in that part, in both valves. Mediterranean. 2 Spe- 
cies. Pl.v. Fig. 39. 
3d Family. 
LitrHorpHaGa. (contains 3 genera.) 
Boring shells, without any accessary pieces, or sheath, and 
more or less gaping at the anterior side. Ligament of the 
valves external. 
The animals of these shells pierce calcareous rocks, in which 
they live. The particulars of their organization are unknown. 
The posterior side of the shell is short, rounded, or obtuse; 
the ligament of the valves, always external. Fleuriau de Belle- 
vue thinks that the boring shells do not pierce stones mecha- 
nically, but by means of a solvent liquid, secreted by the 
animal +. 
1. Saxicava f. 
Shell bivalve, transverse, inequilateral, gaping anteriorly at 
the upper margin. Hinge almost without teeth. Ligament 
external; sometimes the hinge has two distant, scarcely denti- 
form tuberosities. The shells are short and obtuse posteriorly, 
anteriorly longer, flatter, and often truncated. They are small, 
or of moderate size. 
Type. Saxicava rugosa§. (Mytilus rugosus. Linn.) 
Shell rough, ovate, obtuse at both ends, transversely striated. 
North Sea. 4 Species. Pl. v. Fig. 40. 
2. Petricola ||. 
Shell bivalve, subtriangular, transverse, inequilateral ; pos- 
terior side rounded; anterior attenuated, slightly gaping. 
Hinge with two teeth on each valve, or only on one. 
* Beaked. + Avery improbable supposition. 
¢ From saxum, a stone, and cavo, to hollow. § Wrinkled, 
|] From petra, a rock, and cole, to inhabit. 
