1909.] NATURAL SCIENCES OF PHILADELPHIA. 521 



Paohycheilus pila n. sp. PI. XXIV, figs. 1-5. 



The shell is solid and strong, irregularly globose, black, smooth 

 but for inconspicuous growth-lines (except the very young shells 

 which are delicately, closely striate spirally); spire very short, mainly 

 eroded in adult shells, but concavely acuminate when preserved in the 

 young. Only 2 or 3 whorls remain in adult shells, the last very globose 

 throughout, more or less flattened below the suture near the aperture. 

 The aperture is large, black within; outer Up thin, much more arcuate 

 than the inner lip, which is only slightly arcuate. Columellar margin 

 heavily calloused throughout, black, with a white lining on the inner 

 border. 



Length 19, diam. 16^ mm., longest axis of aperture 13 mm. 

 " 20, " 15£ " " " " 13 " 



19, " 17 " aperture 13£ x 12£ mm. 



20, " 17 " " 13x10 mm. 



Tamosopo River above and below the "Natural Bridge,'" near 

 Verastagu, State of San Luis Potosi, Mexico. 



The water in the basin above the bridge is in constant agitation, 

 caused by the falls of several small streams feeding it and the narrow 

 outlet, so there are several inches of the rock walls alternately covered 

 and exposed. The positions occupied by the different species of 

 Pachycheilus were quite diverse, the P. pila always being submerged, 

 while P. atratus and humerosus were higher up and often entirely 

 beyond the reach of the water. Below the bridge P. pila was on 

 submerged rocks, while the other species were most plentiful at or near 

 the surface. 



This is the shortest, most globose species of Pachycheilus known, 

 having the contour of an Anculosa or Paludomus. The youngest stage 

 examined is about 2 mm. long, at which time the shape is ovate-conic. 

 At 5^ mm. long, with the spire of 5 whorls still perfect, the last whorl 

 has become inflated, and traces of very fine delicate spiral strise 

 remain. The types (pi. XXIV, figs. 1 to 5) are from below the "Nat- 

 ural Bridge." Some examples apparently adult are smaller than the 

 dimensions given above, alt. 13^, diam. 13 mm. 



Specimens from a tributary of the Tamosopo River at the Tamosopo 

 Sugar Company's plantation are all smaller. Specimens measure: 

 Length 14, diam. 12, length of aperture 9 mm. 

 " ' 12, " 11, " " 8.8 " 



Examples from a rill on the mountain side above the banana planta- 

 tion at the "Natural Bridge" are very small, adults measuring 8 to 11 

 34 



