168 BULLETIN 68, UNITED STATES NATIONAL MUSEUM. 



constricted at tlic sutures and appressed at the summits, marked 

 by six spiral cords between the sutures, of which the second and 

 third below the summit are very narrow, occupying together about 

 as much space as one of the other cords. These cords are separated 

 by grooves which almost equal them. Axial sculpture reduced to 

 feeble indications of ribs which are best shown near the summit of 

 the whorls, where they render the spiral keels feebly nodulous. About 

 twenty-two of these ribs appear upon the penultimate whorl. Periph- 

 ery and base of the last whorl well rounded, marked by seven spiral 

 keels, which grow successively weaker from the periphery to the um- 

 bilical area. Channels separating the cords narrow, well incised, 

 crossed by numerous slender axial tlireads. Aperture large, broadly 

 ovate, posterior angle acute; outer lip thin; columella moderately 

 strong, ciu-ved, slightly reflected; parietal wall glazed with a thin 

 callus. 



The type (Cat. no. 162775, U.S.N.M.) comes from San Diego, Cali- 

 fornia. It has four post-nuclear whorls and measures: Length 1.8 

 mm., diameter 1.1 mm. 



ODOSTOMIA (CHRYSALLIDA) ROTUNDATA Carpenter. 

 Plate 18, fig. 4. 

 Chrysallida rotundata Carpenter, Oat. Mazatlan shells, 1856, p. 418. 



Shell ovate. Nuclear whorls two and one-half, forming a depressed 

 helicoid spire whose axis is at right angles to that of the succeeding 

 turns. Post-nuclear whorls well rounded, moderately contracted at 

 the sutures, very slightly shouldered at the summit, marked by five 

 strong spiral keels on all the wdiorls between the sutures, excepting 

 the first which has four and obsolete axial ribs on the first two. 

 These axial ribs are best expressed near the summit of the whorls, 

 scarcely reacliing the suture, and rendering the spiral cords feebly 

 tuberculate. On the last whorl the axial sculpture is reduced to 

 numerous raised axial threads, hke those between the cords on the 

 base. Suture poorly defined. Base of the last whorl well rounded, 

 marked by six spiral cords of which the two anterior ones are a little 

 weaker than the rest; separated by spaces which are a Kttle narrower 

 than the cords and crossed by numerous fine axial threads. Aperture 

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

 sculpture within; columella stout, curved, provided with a weak 

 fold at its insertion; parietal wall covered with a thin callus. 



Nine complete shells and a few fragments were found on Spondylus 

 at I^Iazatlan, Mexico. Tablet 1970, Liverpool collection, British 

 Museum, contains two specimens. One of these has four post- 

 nuclear whorls and measures: Length 2.3 mm., diameter 1.1 mm. 



