180 BULLETIN ISl, inSTIT'EiD STATES NlATTOfNlAOL. MUSEIUM 



KEY TO THE SPECIES OF MEXCYCLOTUS 



Shell helicoid cooperi 



Shell depressed-helicoid lutescens 



MEXCYCLOTUS COOPERI (Tryon) 



Plate 24, Figures 10-12 



1863. Cyclotus cooperi Tryon, Proc. Acad. Nat. Sci. Philadelphia, 1863, p. 281, 



pi. 2, fig. 2. 

 1890. Cyclophorus {Amphicyclotus) lutescens von Martens (in part), Biologia 



Centrali-Americana, p. 7. 



1897. Cyrtotoma lutescens var. cooperi Kobelt and Mollendokff, Nachrb. 



deutschen malak. Ges., vol. 29, p. 140. 



1898. Amphicyclotus lutescens var. cooperi Kobelt and M^llendorff, Nachrb. 



deutschen malak. Ges., vol. 29, p. 139 (reprint). 

 1928. Aperostoma cooperi H. B. Baker, Occ. Pap. Mus. Zool. Univ. Michigan, 

 No. 193, p. 53. 



Shell elevated helicoid, covered with a thin pale orange perios- 

 traciim, which shows slender, distantly spaced, and irregularly dis- 

 tributed spiral threads of a brighter yellow on the spire. Interior 

 of aperture yellowish white. Nuclear whorls 1.2, well rounded, 

 forming a small apex. Postnuclear whorls inflated, strongly rounded, 

 and marked by rather distantly spaced, slender, retractively curved 

 axial riblets, which become somewhat irregular on the last turn. 

 Suture strongly impressed. Periphery inflated, well rounded. Base 

 well rounded, openly umbilicated, with the umbilicus about one-fourth 

 the diameter of the shell, marked by the enfeebled ribs, which here 

 constitute merely irregular incremental lines. These also pass over 

 the umbilical wall. Aperture slightly oblique, circular, feebly angu- 

 lated at the posterior angle of the aperture; peristome very thin 

 on the outer lip and considerably thickened on the inner lip. Oper- 

 culum typical. 



The type, Acad. Nat. Sci. Philadelphia No. 10019, was collected by 

 August Eemond near Mazatlan, Sinaloa, Mexico. It has 4.8 whorls 

 and measures: Height, 14 mm.; greater diameter, 16 mm.; lesser 

 diameter, 12.8 mm. 



U.S.N.M. No. 251718 contains 12 additional dead shells, some of 

 which are considerably larger than the type. One of these has 3 

 whorls remaining and measures: Height, 14 mm.; greater diameter, 

 17.8 mm. ; lesser diameter, 14.5 mm. Still another specimen, U.S.N.M. 

 No. 251717, was collected by Dr. E. W. Nelson at San Sebastian, 

 Jalisco. Dr. Nelson also collected a series of specimens at Ixtapan, 

 Jalisco, Mexico. While there is considerable variation in the size of 

 these, they agree quite well with Tryon's type. 



This species can readily be distinguished from M. lutescens 

 (Pfeiffer) by its much more elevated form and stronger ribbing. 



