1909.] NATURAL SCIENCES OF PHILADELPHIA. 531 



rill on the road to the Mocteziuna River. The shells were found where 

 there was little or no current and seemed to be feeding on decaying 

 vegetation, chiefly on fruit of the wild orange. 



This species is distinguished from others of its group by the deep 

 Pleurotomoid notch. In Pachycheilus this structure is known only 

 in P. dalli Pils., a species very different in other characters. Perfect- 

 shells may have as many as 12 whorls, but the tip is generally Inst in 

 adults. 



Explanation of Plates XXIII. XXIV. 



Plate XXIII. — Figs. 1-5 — Pachycheilus pila n. sp. Cotypes. 

 Fig. 6 — P. pila pilula n. subsp. Cotype. 

 Figs. 7-10 — Pachycheilus trisHs n. sp. Cotypes. 

 Figs. 11, 12 — Pachycheilus moctezumensis n. sp. Cotypes. 

 Figs. 13-15 — Pachycheilus pleurotoma n. sp. Cotypes. 

 Figs. 16-20 — Pachycheilus suturalis n. sp. Cotypes. 

 Figs. 21-25 — Pachycheilus monachus n. sp. Cotypes. 



Plate XXIV.— Figs. 1-5— Pachycheilus pluristriatus (Say). Coy River. 

 Figs. 6-10 — Pachycheilus pluristriatus tomasopensis n. subsp. Cotypes. 

 Figs. 11, 12 — Pachycheilus pluristriatus longus n. subsp. Cotypes. 

 Figs. 13-15 — Pachycheilus atratus n. sp. Cotypes. 

 Figs. 16, 17, 18 — Pachycheilus atratus n. sp. Los Canoas. 

 Figs. 19, 20 — Pachycheilus atratus ganinus n. subsp. Cotypes. 

 Figs. 21-25 — Pachycheilus humerosus n. sp. Cotypes. 



