226 NEW-YORK FAUNA MOLLUSCA. 



P. abruptum. (Haldeman, Proc. Ac. Sc. 1841.) Shell ovate: beaks nearly terminal, whence the 

 outline slopes rather abruptly towards the ventral margin. Color, olivaceous or chesnut. Elk 

 river, Maryland. 



P. abditu.m. (Id. lb.) Shell small, ovate, rather elevated, ventricose: beaks in contact. Color, light 

 ochraceous. Springs, Lancaster County, Pennsylvania. 



Genus Cyrena, Lam. Animal with the lobes of the mantle united at their posterior third, and pro- 

 longed there by two siphons separated to their base. Shell solid, subtrigonal or suborbicular, 

 turgid and ventricose. Hinge with three teeth in each valve. Lateral teeth two, one of which 

 is near the primary ones. Ligament exterior, a great part of which is inserted. 

 C. caroliniensis, Bosc. (Say, Nich. Ency. Pl. 25, fig. 266 of this book.) Shell cordate, turgid: 

 surface with numerous membranaceous wrinkles; umbo much eroded ; beaks distant; two of the 

 primary teeth canaliculate at tip ; lateral anterior tooth most elevated ; cavity profound. Color : 

 epidermis olive brown ; within salmon-colored ; purplish on the margins. Length, I •2; breadth, 

 1-3; diameter, 1*1. Carolina. 



FAMILY SAXICAVID^. 



Shell burrowing, without accessory valves, and more or less gaping at the anterior extremi- 

 ties : ligament external. Marine. 



Obs. This corresponds with the Lithophages of Lamarck ; a family remarkable for their 

 general propensity to imbed themselves in calcareous rocks, or in hardened clay beds, in 

 such a manner that their anterior extremities always project outwardly. The manner in which 

 this is effected is not yet ascertained. As it is exclusively in rocks of a chalky nature that 

 they have been found, it has been concluded that an entrance must have been made by an 

 acid secretion which would dissolve the rock. This reasoning would not apply to the cases 

 where they are found, as in this country, in an indurated clay or peat bed. In this country, 

 two genera have been observed. 



GENUS SAXICAVA. Fl. de Bellevue. 



Animal with the mantle closed all round, prolonged behind into a long tube which is double 

 within, slightly divided at its summit, and pierced in front with a rounded aperture for the 

 passage of a small slender lengthened and pointed foot. Mouth moderate ; labial appen- 

 dages small. Branchial plates free for the most part, and very unequal on the same side. 

 Shell transverse, inequilateral ; the anterior upper margin gaping. Hinge nearly without 

 teeth ; ligament external. 



