THE CYOLOPHO'ROD LAND MOLLUSKS OF AMEIRIOA 195 



This species in size resembles A. (/.) pizart^oi. It has, however, a 

 comparatively smaller aperture ; it differs from A. (/.) nirafe in being 

 larger and in having the axial riblets much more regular and closely 

 spaced. 



APEROSTOMA (INCIDOSTOMA) STIRLINGI, new species 



Plate 27, Figxjres 20-22 



Shell helicoid, the upper surface very dark olive; below the periph- 

 ery there is a broad, almost black band, which gradually fades into 

 dark olive. This in turn becomes paler toward the umbilical wall. 

 There is a pale zone immediately above the periphery. Interior of 

 the aperture bluish white. The nucleus consists of 2 well-rounded 

 smooth whorls. The postnuclear whorls are slightly flattened toward 

 the suture; the rest are well rounded. The postnuclear whorls are 

 marked by rather regular, slender, retractively curved riblets, which 

 are about as wide as the spaces that separate them. Suture moder- 

 ately impressed. The periphery appears to be weakly angulated, but 

 this is an optical illusion owing to the light colored band. Base nar- 

 rowly umbilicated for the group, marked by the continuation of the 

 axial riblets, which gain slightly in strength on the umbilical wall. 

 Aperture almost circular ; peristome simple, the inner lip slightly thick- 

 ened, the outer acute, drawn out into a slight auricle at the posterior 

 angle. This protraction of the outer lip at the posterior auricle 

 produces a ridge that extends back halfway over the whorl on the 

 upper surface of the turn near the suture. Operculum typically 

 aperostomid, showing 8 whorls. 



The type, U.S.N.M. No. 516296, was collected by M. W. Stirling at 

 Mendez, Upper Paute Eiver, Ecuador. It has 4.4 whorls and meas- 

 ures: Height, 15.9 mm.; greater diameter, 25 mm.; lesser diameter, 

 17.9 mm. Height of aperture 11 mm. ; diameter, 11.2 mm. 



U.S.N.M. No. 426538 contains 17 topotypes from the same source. 



The small size will readily distinguish this species from all the 

 other Incidostomas. 



Subgenus Austrocyclotus Bartsch (antea. p. 132) 



Aperostomine shells of helicoid shape, the outside surface of 

 which is marked by closely placed threads crossing each other in 

 protractive and retractive series, producing an engine-turned pattern. 



Type: Aperostoma {Austrocyclotus) stramineum (Reeve). 



Distribution: Panama to Ecuador and Venezuela and the West 

 Indies. 



According to Bland, the radula formula of A. (Austrocyclotus) 

 stramineum (Reeve) is 3 : 3 : 3 : 2. The verge in A. (A.) aulari (H. B. 



