166 BULLETIN 68, UNITED STATES NATIONAL MUSEUM. 



which practically terminate at the posterior edge of the suprape- 

 ripheral spiral keel. Of these ribs, 12 occur upon the first, 14 upon 

 the second, IS upon the third, and 22 upon the penultimate turn. 

 In addition to these ribs the whorls are marked by four spiral keels 

 between the sutures which equal the ribs in strength and render the 

 three anterior to the summit tuberculated at their junction, the 

 fom-th one being smooth. Sutures strongly channeled. Periphery of 

 the last whorl marked by a groove. Base well rounded, marked by 

 six slender spiral keels, which are a little less strongly developed and 

 more closely spaced at the umbilical area than at the periphery. 

 Grooves separating these keels, about twice as wide as the keels, 

 crossed by slender axial threads which correspond to the ribs. Aper- 

 ture ovate, somewhat effuse anteriorly; posterior angle obtuse; outer 

 lip sinuous, thin, showing the external sculpture within; columella 

 moderately strong, reflected, provided with a slender fold at its in- 

 sertion; parietal wall glazed with a thin callus. 



Twent}^ specimens were taken off Spondylus at Mazatlan, Mexico. 

 Tablet 1981, Liverpool collection, British Museum, contains five speci- 

 mens, one of which has five post-nuclear whorls and measures : Length 

 2.1 mm., diameter 1.1 mm. 



ODOSTOMIA (CHRYSALLIDA) HELGA, new species. 



Plate 17, figs. 8, 8a. 



Shell conic, milk-white. Nuclear whorls smooth, deeply obliquely 

 immersed in the first of the succeeding whorls, above which only a 

 portion of the last two volutions project. Post-nuclear whorls moder- 

 ately rounded, slightly contracted at the sutures, feebly shouldered 

 at the summits, marked between the sutm-es by fom- broad low spiral 

 bands which are separated by narrow, deeply incised lines. In addi- 

 tion to these bands, the first tliree and one-half whorls are marked by 

 feeble axial ribs which are best developed near the summit of the 

 whorls and scarcely reach the sutm-e. The junction of the ribs and 

 cords form weak nodules. Sutures strongly impressed but not chan- 

 neled. Periphery of the last whorl well rounded, marked by a low 

 spiral cord. Base stronglyrounded posteriorly, attenuated anteriorly, 

 marked by seven spiral cords, which grow successively weaker from 

 the periphery toward the umbilical region and are separated by 

 slencler, deeply-incised spiral lines. Apertiu-e large, broadly oval; 

 posterior angle acute; outer lip thin, showing the external sculpture 

 within; columella moderately strong, cm-ved, reflected, and reinforced 

 by the base; provided with an oblique fold at its insertion. 



The type (Cat. no. 60905, U.S.N.M.) and ten specimens come from 

 San Diego, California. The tj^^e has six post-nuclear whorls and 

 measures: Length 4.2 mm., diameter 2 mm. 



