440 petricolidjK. 



Fam. PETKICOLID^E. 



Labial palps small, triangular. Mantle closed below and 

 behind, the edge thickened and reflexed over the margins of 

 the valves, pedal aperture short, anterior ; siphons elongated, 

 unequal, separated as far as the base ; the branchial the 

 larger, cylindrical, the orifice fringed with arborescent fila- 

 ments ; the anal conical, with a membranous valve, and 

 furnished at the margin with a single series of simple cirrhi ; 

 gills unequal, ovately quadrangular, obliquely plicated. 

 Foot small, compressed, clongately conical, furnished with a 

 byssal groove. 



Shell free or boring, gaping, often irregular; valves 

 usually white, or covered with a thin, hard epidermis. Hinge 

 narrow, bi-dentate in each valve. Pallial line deeply 

 sinuated. 



Genus PETR1COLA, Lamarck. 



Shell ovate, white, covered with a hard, thin epidermis, 

 ventricose, anterior side short, posterior gaping. Hinge 

 composed of two primary teeth in each valve, one of which 

 is often obsolete or rudimentary ; lateral teeth none ; liga- 

 ment short, external. Pallial line with a deep, rounded 

 sinus. 



Ex. P. hyalina, Deshayes, pi. 110, fig. 1. Shell, P. 

 lithophaga, Retzius, fig. 1, a, 1, b. 



The siphons of the animal in Petricola are elongated 

 and distinct from each other, with their orifices ciliated ; the 

 mantle-lobes are united with the exception of a small an- 

 terior opening, and the foot is compressed and lanceolate, 



