530 PROCEEDINGS OF THE ACADEMY OF [Nov., 



in the collection of the Academy of Natural Sciences of Philadelphia. 

 In P. saussurei several whorls following the embryonic shell are convex 

 and smooth, not keeled as in P. suturalis; and lower down the whorls 

 have in addition to the snbsutural group of striae a weak longitudinal 

 plication, which is wanting in P. suturalis. 



The largest examples are from a creek about a mile above the ford 

 of the Moctezuma River. Two measure: 



Length 34, diam. 15, aperture 13 mm.; whorls remaining 4. 

 " 33.2, " 12.2, " 12 " " " 7i 



Those from the creek at Coxcantlan are not quite so large as the 

 type lot, and the shells from the two rills in the mountains are small ; 

 those from the first rill have the lip slightly expanded, and are quite 

 slender, measuring: 



Length 18.2, diam. 7, aperture 6.2 mm.; whorls 8^. 



" 20, " 7.3, " 7 " •" 10 (apex perfect). 



They live in the same stream with P. pleurotoma, but were not noticed 

 feeding with that species, seeming to prefer the rocks where there is 

 considerable current. 



Those from the second rill are slightly larger and less slender, about 

 21 x 10 mm. 



Pachycheilus pleurotoma n. sp. PI. XXIV, figs. 13 15. 



The shell is slender, turrited or spire-shaped, rather thin, hair-brown 

 or greenish-olive. Spire long, slightly attenuate above. The earliest 

 whorls are smooth and rounded ; then some spiral threads appear, one 

 near the lower suture rapidly increasing to a projecting carina, with one 

 thread below, one to three above it; this stage continuing 3 or 4 

 whorls. The carina then disappears, leaving the whorls rounded with 

 about 5 threads, and sometimes a low, indistinct longitudinal plication. 

 The spiral threads then disappear, leaving the last 2 to 4 whorls convex 

 and smooth except for 3 or 4 spiral cords just below the suture. The last 

 whorl usually becomes a little shouldered near the aperture. The 

 aperture is small, very broadly ovate, being somewhat narrower but 

 rounded above. The outer lip, close below the suture, is retracted 

 to form a deep rounded sinus, much as in the marine genus Drillia. 

 Elsewhere the lip is thin and simple. The basal lip is broadly rounded. 

 Columellar margin straightened or much less arched than the outer 

 margin, moderately thickened. 



Length 36 J, diam. 12^, length of aperture 1H mm.; whorls 10. 

 « 29, " 10i, « « « 92 u „ 1Q 



Mountain rill on the road going north from Tampamolon, and 

 several small streams not far from Coxcantlan. Types from the first 



