114 PETRICOLID^. 



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. 



SowERBYA, d'Orbigny, 1850. 



Etym. — Dedicated to Sowerby, author of " British Mineral 

 Conchology," etc. Byn. — Isodonta, Buvignier, 1851. 



Distr. — Fossil, 8 sp. Lower Lias — Portlandian ; England, 

 France, Grermany. S. Deshayesii, Buv. (cxii, 93). 



Shell equivalve, subequilateral ; right valve with two oblique, 

 diA^erging, 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 tw^o, longitudinal lamellar and pro- 

 jecting, and united to the superior border; ligament external. 



Family 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, brancliial cirri pin- 

 nate ; palpi small, triangular. 



Petbicola, Lamarck, 1801. 



Etym. — Petra, stone ; colo, to inhabit. 



Syn. — Choristodon, Jonas (in part). 



Distr.^lB sp. United States, France, Red Sea, India, New 

 Zealand, Pacific, West America (Sitka — Peru). Burrows in 

 limestone and mud. Fossil, 20 sp. Cretaceous, Eocene — ; 

 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, 18Y0. For the transversely elongated 

 forms, of which P. pholadiformis, Lam. (cxii, 94), is the type. 

 This species is very common, perforating clay or mud upon the 

 sandy beaches of New Jersey. 



RuPELLARiA, Fl. de Bellevue, 1802. 

 Syn. — Yenerupis, Lara., 1818. 



Distr. — 30 sp. Europe, Pacific, etc. Fossil; Jurassic — . R. 

 foliacea^ Desh. (cvii, 96). 



Shell elongated, moderately tumid, surface rugosely striated 



