WEST AMERICAN PYRAMIDELLID MOLLUSKS. 



173 



with three spiral keels between the sutures, one of which at the 

 summit is slender, the other two are strong and equal, the supra- 

 peripheral one being about as far posterior to the suture as the one 

 at the summit is fi"om its neighbor. In addition to the spiral keels 

 the whorls are marked by narrow retractive axial ribs, of which 14 

 occur upon the first, 16 upon the second to third, 18 upon the fourth, 

 20 upon the fifth and sixth, and 24 upon the penultimate turn. 

 The junctions of the axial ribs and spiral keels are somewhat tuber- 

 culated, while the spaces inclosed between them are deeply impressed 

 pits. A strong keel marks the periphery of the last whorl and 

 another equally strong occupies the middle of the base, the space 

 between them being a concave channel, which, like the one posterior 

 to the peripheral keel, is crossed by the axial ribs. The axial ribs 

 become much enfeebled as they pass over the basal keel and are 

 almost obsolete on the spaces anterior to it. Aperture irregularly 

 oval; posterior angle obtuse; outer lip thin, rendered angular by the 

 spiral keels; columella very strong, almost straight, slightly reflected; 

 parietal wall covered by a thin callus. 



The type (Cat. no. 107422, U.S.N.M.) comes from San Pedro, Cali- 

 fornia. It has eight post-nuclear whorls, and measures: Length 

 6.7 mm., diameter 2.5 mm. 



We have examined the following specimens: 



ODOSTOMIA (IVIDELLA) NAVISA Dall and Bartsch. 

 Plate 18, figs. 11, 11«. 



Odostomia (Ividia) navisa Dall and Bartsch, Proc. U. S. Nat. Mus., vol. 33, 

 1907, pp. 517, 518, pi. 46, figs. 2, 2a. 



Shell of medium size, strongly sculptured, subdiaphanous to milk- 

 white. Nuclear whorls at least two, obliquely a little more than half 

 immersed. Post-nuclear whorls strongly shouldered, subtabulated, 

 with a strong, broad, spiral keel limiting the anterior edge of the 

 shoulder and an acute raised keel on the middle of the whorls 



