262 BULLETIN 181, UNITED STATES NATIONAL MUSEUM 



slanting axial riblets. Suture strongly impressed. Periphery ob- 

 scurely angulated. Base moderately broadly umbilicated, inflated, 

 well rounded, with the axial riblets growing a little stronger on the um- 

 bilical wall. Aperture subcircular ; peristome simple ; outer lip slightly 

 expanded and reflected; inner moderately thickened. The parietal 

 wall is covered by a rather heavy callus, which is separated from the 

 preceding turn by an impressed line. 



The type, U.S.N.M. No. 322360, was collected by H. M. Curran on 

 the Rio Gregugy, Bahia, Brazil. It has a little more than 2 whorls 

 remaining and measures : Height, 16 mm. ; greater diameter, 27.5 mm. ; 

 lesser diameter, 10.8 mm. Height of aperture, 12.1 mm. 



We dislike to describe imperfect material as new species but are 

 making an exception in this case because the specimen in question 

 represents an element completing the range of the group to which 

 it belongs. 



The species can be easily differentiated from A. (A.) mejTilU by 

 its larger size. 



APEROSTOMA (APEROSTOMA) NANUM, new species 



Plate 38, Figures 13-15 



Shell small, helicoid ; the upper surface horn yellow with a narrow 

 light zone immediately above the periphery. Below the periphery 

 there is a narrow blackish-brown zone. The rest of the base is of the 

 same color as the upper surface. The nucleus consists of 2 well- 

 rounded, smooth turns. The postnuclear whorls are well rounded ; the 

 last one has an inconspicuous cord at the summit immediately adjacent 

 to which is an impressed groove. The summit of the last half turn of 

 the last whorl is gradually deflected until it falls below the dark sub- 

 peripheral band. The postnuclear whorls are marked by retractively 

 slanting axial riblets, which are about as wide as the spaces that sep- 

 arate them. Suture very strongly impressed. Periphery obscurely 

 angulated. Base narrowly umbilicated, inflated, well rounded, marked 

 by the continuation of the axial ribs, which become very much enfeebled 

 on the last half. These axial riblets become stronger on the umbilical 

 wall. Aperture circular, slightly angulated at the posterior angle; 

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

 narrow parietal wall is covered by a thick callus which renders the 

 peritreme complete. 



The type, U.S.N.M. No. 336128, was collected by H. Pittier on the 

 coastal range somewhere between sea level and 200 meters between 

 Caracas and Puerto Cabello, Venezuela. It has 4 whorls and meas- 

 ures : Height, 13.8 mm. ; greater diameter, 21.7 mm. ; lesser diameter, 

 15.3 mm. Height of aperture, 8.5 mm. ; diameter, 10.1 mm. 



