220 BETUSA. 



about two turns is usually visible; this margin, formed by the rather 

 broad P -shaped posterior sinus of the aperture,.resembles the notch- 

 band of some Pleurotomidse in that the surface is flattened, with a 

 well-marked boundary on each side, and on this surface the succes- 

 sive marginal edges are often raised into scales, one fitting into 

 another, composed of an extension of the body callus on one side 

 and a reflection of the free margin on the other; the surface of the 

 band varies in different specimens from nearly smooth to distinctly 

 and regularly undulated or imbricately scaled as above mentioned ; 

 other transverse sculpture of lines of growth which are hardly visible 

 while of spiral sculpture there is none, though, with a strong reflected 

 light, under the microscope numerous spiral markings may be 

 observed which are neither grooved nor raised, but are visible in 

 most smooth spiral shells, and are probably due to growth, somewhat 

 as are the lines commonly recognized as "lines of growth." Aper- 

 tture nearly or quite as long as the shell, narrow, rounded in front, 

 and terminating in the P -shaped sinus behind ; outer lip straight, 

 sharp, thin, not incurved, rounded to join the stout columella into 

 which it passes imperceptibly; pillar broad, short, with a thin callus 

 which also extends along the body ; shell widest about the posterior 

 third ; distinctly narrowed anteriorly. Lon. of shell and aperture 

 (the latter occasionally a trifle less), 8*2. Max. lat. of shell (at pos- 

 terior third), 4:0; at anterior third, 3'5 ; of aperture, 1*75; min. 

 lat. of aperture, 0'5 mill. (Dall). 



Off Cape San Antonio ; Yucatan Strait, 640 fins. 



Utrieulu8ffrieleiVKL, Bull. M. C. Z. ix, p. 104, 1881 ; Blake 

 Oastr., p. 47, pi. 17, f. 4. 



Utriculus leucus Watson seem to approach this species as nearly 

 as any known form, but has sundry distinctive characters. There 

 is no doubt, however, that there are differences of form and develop- 

 ment of the tip of the spire in these enrolled forms, in adult individ- 

 uals, as well as during the stages of one individual. It will not do, 

 therefore, to draw the specific lines too taut on this sort of character. 

 (Dall). 



R. PERVIUS Dall. Unfigured. 



Shell short, stout, truncate apically, white, polished, sculptured 

 only with faint incremental lines ; form subcylindrical, larger ante- 

 riorly, a little compressed just behind the middle ; aperture long, 

 narrow behind and rounded at the posterior commissure, where it 



