522 PROCEEDINGS OF THE ACADEMY OF [Nov.. 



mm. long. This small race might be distinguished as P. pila pilula 

 n. var. (pi. XXIV, fig. 6). 



Pachycheilus tristis n. sp. PI. XXIV, figs. 7-10. 



The shell is short, ovate, brown, the last whorl smooth when adult. 

 Spire conic but deeply eroded in adults, rarely showing more than 

 3 whorls distinctly. The last whorl is convex. Aperture bluish-white 

 inside. Outer margin thin, strongly arched, forming half a circle; 

 inner margin slightly arcuate, heavily calloused, white throughout 

 or with a brown edge. 



The neanic stage, length 10-11 mm., is conic with an elevated spire 

 sculptured with spiral carinse, 3 on each whorl of the spire, one ridge 

 being just below the suture, another below the middle, and the third 

 just above the succeeding suture. On the last whorl there are 4 or 5 

 spiral ridges, the lowest one weak, below the periphery. Subsequently 

 the whorls become more inflated and the spiral sculpture weakens, 

 either disappearing in the penultimate whorl or a single subsutural 

 ridge and sulcus may persist until the beginning of the last whorl. 



Length 21, diam. 13.7 mm.; aperture 11.5 x 9 mm. 

 20, " 14 " " 11.2 x 9.2 " 



23.5, " 16 " " 12.5x10 " 



16.2, " 12.5 " " 9.5 x 8 " 



Tamasopo River at the "Natural Bridge," near Verastagu, State of 

 San Luis Potosi. Very few found above the bridge, but quite plentiful 

 below. 



Adult shells of this species resemble P. pila in shape, but they differ 

 by the brown color, bluish-white aperture and white columella. Old 

 shells which have lost most of the spire by erosion are hard to dis- 

 tinguish from P. pila. The characters of the young are wholly diverse, 

 and show P. tristis to be related to P. humerosus and P. atratus. Com- 

 pared with P. humerosus, P. tristis differs in lacking a shouldered last 

 whorl, the more ovate shape and the color. It differs from P. atratus 

 in shape, sculpture and color. 



Pachycheilus moctezumensis n. sp. PL XXIV, figs. 11, 12. 



The shell is globose-oval, solid, purplish-black. The spire is very 

 short, the apex slightly projecting; earlier whorls light brown, the apex 

 eroded. Whorls of the spire sculptured with several low spiral ridges. 

 Last whorl oval, smooth, aperture ovate, angular above, 'broadly 

 rounded below, dark within, black near the lip edge. Inner margin 

 calloused, white, usually with a brown edge. 



