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PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. XXXII. 



weaker than the rest. Aperture decidedly oblique, polygonal, the 

 angles being formed by the spiral keels, the posterior angle of the 

 aperture, and the insertion of the stout and strongly curved columella. 

 The unique type, Cat. No. 4035, U.S.N.M., was collected by J. Xan- 

 tus at Cape St. Lucas, Lower California. It has little more than 

 four whorls and measures: Greater diameter 1.4 mm.; lesser diameter 

 1.15 mm.; altitude 0.5 mm. 



CYCLOSTREMA DIEGENSIS, new species. 



Shell exceedingly small, thin, planorboid, with a prominent, com- 

 pressed peripheral keel, translucent, yellow horn-colored. Nepionic 

 whorls li, moderately rounded, not elevated, smooth. The suc- 

 ceeding turns have their highest elevation at about one-third of 

 the distance from the suture to the periphery, at which place they 

 are raised into a broad, well-rounded ridge from which they slope 

 abruptly, convexly rounded, to the suture and more gently concavely 

 to the angulated periphery. On the upper surface the whorls are 

 ornamented with slender, regularly spaced, oblique riblets, which are 



Fig. 7. — CYCLOSTREMA DIEGENSIS. a, TOP VIEW; b, SIDE VIEW: C, BASAL VIEW. 



best developed on the elevated ridge, where they are about one-fourth 

 as wide as the spaces that separate them. Sutures strongly marked. 

 Periphery with a compressed, olituse angle. Base moderatel}" rounded, 

 broadly openly umbilicated, with a slender thread bounding the outer 

 edge of the umljilicus and a slender spiral cord situated about halfway 

 between the umbilical thread and the periphery. The riblets seen on 

 the upper surface extend feebly beyond the peripheral keel to the first 

 basal cord but are reduced to simple incremental lines between this 

 and the umbilical thread; columellar wall well rounded, marked by 

 incremental lines only. Aperture oblique, irregularly pentagonal, 

 one blunt angle being formed by the elevated part of the whorl, another 

 equally obtuse one by the periphery, the third by the basal keel, the 

 fourth by the umbilical angle, and the fifth by the junction of the 

 columella with the parietal wall. Outer lip thin, showing the sculp- 

 ture of the shell within. Columella straight and very obliquely placed. 

 Parietal wall covered by a thin callus. 



There are five specimens of this species in the collection of the U. S. 

 National Museum, Cat. No. 105488, all from San Diego, California. 



