204 BULLETTN 181, XINITEiD SWATEI9 NlA'TTOfMAiL MUSEtUM 



upper surface, while the anterior half has the riblets more irregular 

 and less nodulose. On the umbilical wall the axial riblets become 

 somewhat intensified both in strength and irregularity. Aperture 

 subcircular, slightly auriculated posteriorly. Outer lip of the peri- 

 stome thin, the inner slightly thickened. The parietal wall is cov- 

 ered by a moderately thick callus. Operculum typically neocyclotid. 



The type, U.S.N.M. No 536033, was collected by Dr. Alexander 

 Wetmore and M. A. Carriker, Jr., at Tierra Nueva, Sierra Negros, 

 Magdalena, Colombia. It has 3.4 whorls remaining and measures: 

 Height, 22.6 mm.; greater diameter, 28.7 mm.; lesser diameter, 

 21.4 mm. Height of aperture, 15.4 mm. ; diameter, 15.0 mm. 



The jaw is typically aperostomid, and the radula has the formula 

 3:3:3:2. 



U.S.N.M. No. 536034 contains six topotypes. 



This species resembles A. (N.) smithi Pilsbry and Clapp, from 

 which it can readily be distinguished by its very narrow umbilicus. 

 From the dysoni group it can be distinguished by its broadly flattened 

 shoulder at the summit. 



APEROSTOMA (NEOCYCLOTUS) DYSONI (Pfeiffer) 



Shell helicoid, varying greatly in size in the different races, covered 

 with a thin periostracum, which varies materially in color in the dif- 

 ferent subspecies ; it may be unicolor or bicolor, that is the base may 

 be of a different shade from that of the spire. The nuclear turns are 

 about 2, well rounded, smooth. The postnuclear whorls are well 

 rounded and marked by vermiculations, which also vary greatly in 

 strength in the different forms. The base is well rounded, openly 

 umbilicated, the umbilication varying in width in the different races. 

 The umbilical wall bears axial riblets that also vary in strength with 

 the subspecies. Aperture almost circular; peristome simple, that of 

 the inner lip usually thickened. Operculum tj'^pically neocyclotid. 



The following key will help to differentiate the subspecies: 



KEY TO THE SUBSPECIES OF APEROSTOMA (NEOCYCLOTUS) DYSONI 



Shell gigantic; greater diameter raore than 33 mm dyeri 



Shell not gigantic ; greater diameter less than 30 mm. 

 Axial sculpture rather strongly vermiculated. 

 Vermiculations fine and closely spaced. 



Shell large; greater diameter more than 27 mm hinkleyi 



Shell small; greater diameter less than 22 mm ruatanense 



Vermiculations not fine or closely spaced. 



Whorls somewhat flattened toward the summit. 



Greater diameter more than 25 mm dysoni 



Greater diameter less than 20 mm aureum 



Whorls not flattened toward the summit. 



Columellar tip thickened sumichrasti 



Columellar tip not thickened affine 



