162 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.58. 



whorls decidedly depressed, the first almost completely immersed in 

 the second. Postnuclear whorls flattened axially and spirally, 

 minutely striated. Suture very deep, owing to the shoulder of the 

 whorls and the peripheral angle. Periphery decidedly angulated. 

 Base short, moderately rounded with a broad spiral color band on 

 its middle. Aperture subcircular; outer lip thin; inner lip well 

 rounded ; parietal wall covered by a thick callus. 



The type was described as from Panama Bay. The measurements 

 given for it are: Altitude, 2.5 mm.; diameter, 1.4 mm. 



RISSOELLA BIFASCIATA (Carpenter). 



Plate 12, fig. 2. 



1856. Jeffreysia bifasciata Caepenter, Cat. Maz. Shells, p. 362. 



1857. Jeffreysia bifasciata Caepentek, Rept. Brit. Ass. Adv. Sci., 1856, pp. 



257, 327, 366. 

 1864. Jeffreysia bifasciata Caepenter, Rept. Brit. Ass. Adv. Sci., 1863, p. 

 623. 



Shell minute, ovate, very thin, semitranslucent. Nuclear whorl 

 apparently not differentiated from the succeeding turns by sculptural 

 characters or resting mark. Postnuclear whorls inflated, strongly 

 rounded and roundly shouldered at the summit, marked by fine re- 

 tractively slanting lines of growth only. Suture strongly marked. 

 Periphery inflated, strongly rounded. Base short, strongly rounded. 

 Aperture large, oval, posterior angle nearly obtuse; outer lip de- 

 cidedly curved, thin ; inner lip curved and appressed to the base ex- 

 cepting the anterior third, which is free ; parietal wall covered by a 

 thick callus that renders the peristome complete. The shell may be 

 plain white or have a color band or two between summit and suture 

 and one on the base. The bands, when present, are usually only 

 faintly exhibited. 



Cat. No. 56354, U.S.N.M., contains a specimen determined by 

 Carpenter, from Mazatlan, which has five whorls and measures — 

 altitude, 1.2 mm.; diameter, 0.8 mm. Cat. No. 16218, U. S. N. M., 

 contains another species from Cape St. Lucas, Lower California, 

 whose operculum shows the typical thin, pale yellow, horny consist- 

 ency, with the thick ridge on the internal columellar border and the 

 transverse median ridge connecting with this. 



Carpenter writes : 



Of this species, benutifully lustrous when viewed under the microscope with 

 a good light, about 90 specimens were obtained, probably from the Algae on 

 the Uvanillae. They are most likely of somewhat sedentary habits, as even 

 In a living state they are not unfrequently incrusted with Coralline. The 

 dried animals have a rich brown color. Several retained their opercula, which 

 are perfectly normal, and of a reddish brown. Long., 1.375 mm. ; lat. 0.75 mm. 



Hab. Mazatlan ; rare, on ? Algae ; Liverpool Collection [British Museum]. 



