THE CYOLOPHO'RID LiAJSTD MOIiLUSES OF AMBMCLi 231 



Shell depressed-helicoid, the early whorls soiled flesh white; the 

 later turns gradually becoming chestnut brown. There is a broad 

 spiral dark zone covering the posterior half of the base, which grad- 

 ually becomes paler from the periphery basally. The rest of the base 

 is much paler, so that there is a bicolor effect on the base ; interior of 

 aperture decidedly bluish white. The nucleus consists of 2 small, 

 well-rounded, smooth turns. The postnuclear whorls are weakly 

 rounded, the last one flattened a little below the summit, and marked 

 by retractively curved, rather regular axial riblets, which are a little 

 wider than the spaces that separate them. On the last turn, however, 

 these riblets gradually become enfeebled, and on the last portion of this 

 whorl they are merely represented by irregular lines of growth. The 

 last whorl is slightly decurrent on the last fifth of a turn. Suture 

 strongly impressed. Periphery rounded ; the peculiar banding at the 

 periphery, however, gives it the effect of being angulated, which is an 

 optical illusion. Base very openly umbilicated, somewhat inflated, well 

 rounded ; the posterior half is marked by the continuation of the axial 

 ribs which are stronger than on the upper surface, and on the anterior 

 half become fused into strong, distinct, broad, rounded ribs, which 

 cross the sloping umbilical wall. Aperture very broadly tear-shaped 

 with a decidedly drawn-out auricle at the posterior angle, which has 

 an impressed groove in its middle; peristome simple, the outer lip 

 thin; the inner thickened. The parietal wall is covered by a thick 

 callus, which renders the peritreme complete. Operculum typically 

 aperostomid, having 10 whorls. 



The specimen described and figured is one of two, U.S.N.M. No. 

 251170, collected by H. Pittier in the Eio Paila Valley, Colombia, at 

 an altitude of 1,800 meters. It has 4.8 whorls and measures : Height, 

 20.4 mm. ; greater diameter, 38.5 rom. ; lesser diameter, 26 mm. Height 

 of aperture, 13 mm. ; diameter, 17.5 mm. 



The larger size of this species distinguishes it from the other two 

 members here described as having strong ribs on the umbilical wall. 



APEROSTOMA (APEROSTOMA) PAILAENSE, new species 

 Plate 32, Figures 7-9 



Shell quite small, depressed-helicoid. The shreds of periostracum 

 remaining would indicate that this was straw-colored with a gold 

 tinge. The denuded surface is uniformly bluish white; interior of 

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

 smooth turns; the postnuclear whorls are moderately well rounded; 

 the last slightly flattened below the summit, marked by retractively 

 curved slender axial riblets, which are about as wide as the spaces 

 that separate them. On the last half of the last whorl these riblets 



