WEST AMERICAN PYRAMIDELLID MOLLUSKS. 135 



ODOSTOMIA (SALASSIA) SCALARIFORMIS Carpenter. 

 Plate 13, fig. ] . 

 Parthenia scalariformis Carpenter, Cat. Maz. Shells, 1856, p. 413. 



Shell piipiform, white. Nuclear whorls small, almost completely 

 immersed. Post-nuclear whorls well rounded, scarcely at all con- 

 tracted at the periphery, strongly roundedly shouldered at the sum- 

 mit, marked by slender, distant, scalariform, retractive axial ribs, 

 of which about 20 occur upon the first and 16 upon the remaining 

 turns. Intercostal spaces very broad, shallow. Sutures strongly 

 marked. Periphery and the somewhat prolonged base of the last 

 whorl well rounded, marked by the undiminished continuations of 

 the axial ribs, which extend to the umbilical chink. Aperture ovate; 

 outer lip thin; columella slender, curved, provided with a moder- 

 ately strong fold at its insertion; parietal wall covered with a thick 

 callus. 



Two specimens of this species were found on Chama at Mazatlan. 

 Tablet 1962 of the Liverpool collection in the British Museum con- 

 tains the finest of the two. This has six post-nuclear whorls, and 

 measures: Length 5 mm., diameter 0.93 mm. 



Subgenus BESLA Dall and Bartsch. 

 Besla Dall and Bartsch, Proc. Biol. Soc. Wash., vol. 17, 1904, p. 10. 



Small Odostomias with axial ribs and three strong spiral raised 

 threads, one at and two posterior to the periphery between the 

 sutures; base marked by raised spiral threads. 



Type. — Chrysallida convexa Carpenter. 



KEY TO THE SPECIES OP THE SUBGENUS BESLA. 



Whorls overhanging convexa, p. 135. 



Whorls flattened- caUimorpha, p. 136. 



ODOSTOMIA (BESLA) CONVEXA Carpenter. 



Plate 13, fig. 4. 



Chrysallida convexa Carpenter, Cat. Mazatlan Shells, 1856, p. 424. 



Shell small, slender, elongate-conic. Nuclear whorls two and one- 

 half, forming a moderately elevated helicoid spire, whose axis is at 

 right angles to that of the succeeding turns, in the first of which it is 

 about one-fifth immersed. Post-nuclear whorls overhanging, strongly 

 contracted at the sutures, appressed at the summit, angulated at the 

 posterior extremity of the anterior third; between the sutures, 

 marked by strong, rounded, sinuous, almost vertical axial ribs, of 

 which 16 occur upon the second and third, 18 upon the fourth, and 

 22 upon the penultimate turn. Intercostal spaces a little more than 

 twice as wide as the ribs, crossed by three equal and equally spaced 

 spiral cords, which are about one-half as strong as the ribs. The first 



