232 BULLJ^TIN 181, UNITED SWATES NlAmOOSnAiL MUSlETIM 



become much enfeebled and form rather irregular, broad incremental 

 elements. Suture strongly impressed. Periphery with a weak 

 angulation. Base moderately openly umbilicated, inflated, well 

 rounded, and marked by strong axial ribs which gradually increase 

 in intensity toward the umbilicus, being strongest on the sloping 

 umbilical wall. Aperture very broadly oval, with a conspicuous 

 auricle at the posterior angle, which bears a groove in its middle; 

 peristome simple; the outer lip thin; the inner thickened. The 

 parietal wall with a very thick callus rendering the peritreme 

 complete. 



The type, U.S.N.M. No. 251171, was collected by H. Pittier in the 

 Rio Paila Valley, Colombia, at an altitude of 1,300 meters. It has 

 4.3 whorls and measures: Height, 16.3 mm.; greater diameter, 27.8 

 mm.; lesser diameter, 18.9 mm. Height of aperture, 11.7 mm.; 

 diameter, 13.9 mm. 



This species is readily distinguished from the other two with 

 strong ribs on the umbilical wall by its exceedingly small size. 



APEROSTOMA (APEROSTOMA) PAEZICOLUM, new species 



Plate 32, Figures 13-15 



Shell depressed-helicoid, the early whorls reddish; the last whorl 

 olivaceous with axial streaks of brown, which vary in width and 

 spacing. There is a narrow light zone immediately above the 

 periphery. Below the periphery the posterior half of the base has 

 a dark chestnut-brown area; the anterior half is olivaceous brown 

 and much paler. The axial darker streaks mentioned for the upper 

 surface also are apparent on the lower. Interior of aperture bluish 

 white. The nucleus consists of 2.1 small, well-rounded, smooth turns. 

 The postnuclear whorls are moderately well rounded; the last is 

 slightly decurrent on the last half whorl and slightly flattened below 

 the summit. The postnuclear whorls are marked by slender, re- 

 tractively curved axial riblets, which are a little wider than the 

 spaces that separate them. On the last whorl these become less 

 strongly developed and more irregular. There is also a weak indica- 

 tion of a few scratches, showing the affinity of the species to the mal- 

 leated form. Suture strongly impressed. Periphery well rounded. 

 Base openly umbilicated, somewhat inflated, well rounded, and 

 marked by the continuation of the sculpture characterizing the upper 

 surface; the riblets, however, become fused on the umbilical wall 

 to form strong and quite regular rather broad ribs on its sloping 

 surface. Aperture almost circular with a conspicuous auricle at 

 the posterior angle, which bears a groove in its middle; peristome 

 simple; outer lip thin; the inner thickened. The parietal wall is 



