174 PETRICOLIDA. 
long, triangular ; gills unequal, united to the base of the siphons, 
the external pair divided into two nearly equal areas by a longi- 
tudinal furrow, indicating their line of attachment. 
Sowerby, d’Orbigny, 1850. 
Etym.—Dedicated to Sowerby, author of “ British Mineral 
Conchology,” ete. Syn.—Isodonta, Buvignier, 1851. 
Distr.—Fossil, 8 sp. Lower Lias—Portlandian; England, 
France, Germany. 8S. Deshayesi, Buy. (exii, 93). 
Shell equivalve, subequilateral; right valve with two oblique, 
diverging, cardinal teeth separated by a mesial trigonal socket, 
and two lamellar lateral teeth, separated from the hinge-border 
by longitudinal grooves; left valve with a conical tooth between 
two oblique pits; laterals two, longitudinal lamellar and pro- 
jecting, and united to the superior border; ligament external. 
Famity PETRICOLID. 
Shell gaping, free, but frequently perforating clay or soft 
rocks, and therefore often irregular in form; white under a very 
thin epidermis; hinge narrow, bidentate in each valve; sinus of 
the pallial impression profound. 
Animal with the mantle closed in front, much thickened and 
recurved over the edges of the shell; pedal opening small; foot 
small, pointed, lanceolate; siphons partially separate, orifices 
fringed, anal with a valve and simple cirri, branchial cirri pin- 
nate; palpi small, triangular. 
Perrricota, Lamarck, 1801. 
Etym.—Petra, stone ; colo, to inhabit. 
Syn.—Choristodon, Jonas (in part). 
Distr.—13 sp. United States, France, Red Sea, India, New 
Zealand, Pacific, West America (Sitka—Peru). Burrows in 
limestone and mud. Fossil, 20 sp. Cretaceous, Hocene—; 
United States, Europe. 
Shell oval or elongated, thin, tumid, anterior side short ; hinge 
with three teeth in each valve, the external often obsolete; pallial 
sinus deep. 
PETRICOLARIA, Stoliczka, 1870. For the transversely elongated 
forms, of which P. pholadiformis, Lam. (exii, 94), is the type. 
This species is very common, perforating clay or mud upon the 
sandy beaches of New Jersey. 
RvpELLARIA, Fl. de Bellevue, 1802. 
Syn.—Venerupis, Lam., 1818. 
Disir.—30 sp. Europe, Pacific, ete. Fossil; Jurassic—. AR. 
foliacea, Desh. (cvii, 96). 
Shell elongated, moderately tumid, surface rugosely striated 
