Cijdostrema.] GASTROPODA. 153 



Shell small, umbilicated, depressed, white, corneous or transparent, 

 not nacreous ; aperture circular, with continuous sharp peristome ; 

 operculum corneous, multispiral. 



KEY TO GENERA. 



A. Shell depressed, spiral and radial sculpture prominent, um- 



bilicus wide, bounded by a carina . . . . . . CYCLOSTREMA. 



B. Shell depressed, sculpture rather inconspicuous, umbilicus 



wide, without a carina . . . . . . . . DELPHINOIDEA. 



C. Shell turbinate, narrowly umbilicate, peristome continuous, 



double . . . . . . . . . . . . CIRSONELLA. 



D. Shell globose, thick, porcellanous. sub- or im-perforate. peri- 



stome thickened . . . . . . . . . . PSEUDOLIOTIA. 



Genus 1. CYCLOSTREMA, Marryatt, 1818. 



Cyclostrema, Marryatt, Trans. Linn. Soc., xii, 1818, 338. Type : C. canceUata, 

 Marryatt. Cyclostrema, Marryatt, Man. Conch. (1), x, 14, 88. 



Shell depressed, suborbicular, distinctly spirally and radially 

 sculptured, white or uniformly coloured ; aperture nearly circular ; 

 peristome acute, continuous ; umbilicus deep, bounded by a carina ; 

 operculum circular, horny, multispiral, each volution obliquely striated. 



Distribution. Nearly universal. 



Fossil. Tertiary. 



1. Cyclostrema eumorpha, Suter, 1908. Plate 33, figs. 20, a, b. 

 Cyclostrema eumorpha, Suter, P. Mai. 8., viii, 25, pi. 2, f. 7-9. 



Shell very small, turbinate, umbilicate, translucent, white, spirally 

 distantly ribbed, and radiately striate. Sculpture consisting of 5 pro- 

 minent spiral riblets, the first just above the periphery ; a low and 

 indistinct spiral riblet on the last whorl outside the suture, and some- 

 times a fine riblet bordering the funnel-shaped umbilicus ; radiate 

 sculpture formed by distinct threads, which are equidistant, slightly 

 directed backward, the interstices wider than the threads. Colour 

 white. Spire depressed conoidal. lower than the aperture. Proto- 

 conch minute, sphasrical, of 1 whorl only. Whorls 3, regularly in- 

 creasing, convex, the last flattened between the suture and the first 

 spiral riblet, periphery rounded ; base convex. Suture impressed. 

 Aperture oblique, circular. Peristome continuous, smooth inside, orna- 

 mented outside by the spiral sculpture. Columella arcuate, strong, not 

 reflexed. Umbilicus rather narrow, deep. Operculum not seen. 



Diameter Maj., 1-7 mm. ; min., 1-4 mm. : height, 1-3 mm. 



Animal unknown. 



Type in my collection. 



Hob. Snares, in 50 fathoms (Captain Bollons). 



Remark. This species is allied to the South Australian C. de- 

 lectabile, Tate, in which, however, the spiral riblets are more numerous 

 and less prominent ; the umbilicus is much wider, perspective. 



