314 AXEL A. OLSSON 



Family PETKICOLIDAE 



The mollusks of this family are borers in rock, clay, or shale, some- 

 times in thick-walled mollusks, corals, and barnacles. The valves are sub- 

 ovate to elongated in shape, sometimes pholadiform, regular in shape, or 

 much distorted. Hinge venerid, bearing two or three cardinal teeth in each 

 valve of which one or more may be bifid or they are hook-shaped and often 

 enlarged; the posterior cardinal tooth is generally small and atrophied; 

 there are no lateral teeth; the teeth often interlock so closely that the hinge 

 cannot be separated without damage. Ligament external, attached to a 

 coarse nymphal ridge. External sculpture formed by radial riblets and con- 

 centrics. Pallial sinus well developed, extending into the middle of the 

 cavity of the valve. 



This family is represented along the coast of Panama, Ecuador, and 

 Peru, by several species belonging to three groups. Together with other 

 rock borers they are an important factor in shore-line erosion. 



Key to Panamic-Pacific genera 



I. Borers in comparatively hard rocks, hence, their shell usually much 

 distorted or irregular in shape. Hinge teeth variable in development, 

 the posterior left cardinal tooth missing or atrophied in the adult. 

 Ligament deeply immersed. 



1. Surface sculptured by fairly simple, radial riblets; sometimes with 

 concentrics; both elements uniformly developed over the whole disk. 



Genus Petricola 



2. Sculptural pattern more or less divaricate, the radial elements forming 

 disconnected hook-shaped bends or zigzag lines. 



Genus Naranio 



IL Borers in softer rock formations or burrowing in sand, and clay, their 



shells more normal in shape, often elongated, pholadiform, and the 



surface sculpture is regular. Hinge teeth normal, the ligament not 



immersed. 



Genus Petricolaria 



Genus PETRICOLA Lamarck, 1801 

 Type species by subsequent designation. Gray, 1847, Venus lithophagus 

 Retzius. 



Valves of solid texture and generally more or less distorted. Surface 

 sculptured with coarse radial riblets, generally heaviest on the posterior 

 slope, often with concentrics forming together a decussate pattern. The 

 hinge is generally normal in young shells but with growth, the posterior 

 cardinal tooth becomes atrophied and sometimes completely eliminated. 



Petricola (Petricola) denticnlata Sowerby Plate 54, figures 1-ld 



Petricola denticulata Sowerby, 1834, Proc. Zool. Soc. London, p. 46. Hab. ad Paytam 

 Peruviae.— Sowerby, 1854, Thes. Conch., vol. 2, p. 773, No. 6, pi. 146, figs. 

 6, 7, — Sowerby, 1874, Conch. Icon., vol. 19, Petricola, pi. 2, fig. 9. — Pilsbry and 

 Lowe, 1932, Proc. Acad. Nat. Sci. Philadelphia, vol. 84, pp. 97, 98, pi. 13, 

 figs. 1, 2, 3, 3 a. 



Shell narrowly to broadly elongated, variable in shape and sometimes 

 distorted, subsolid. The umbones are wide and prominent, the beaks small, 

 prosogyrous, inrolled slightly over the margin and placed near the anterior 



