182 BULUETI'N ISl, UNlTEiD STATES NlATTONIAiL MUSEIUM 



Type: Megacyclotus ponderosus (Pfeiffer). 

 Distribution : Southeastern Mexico and Guatemala. 

 The radula of M. ponderosus (Pfeiffer) has the formula 3:3:3:2, 

 and the jaw is without a median projection, 



KEY TO THE SPECIES OF MEGACYCLOTUS 



Shell more than 40 mm. in diameter ponderosus 



Shell less than 35 mm. in diameter palenquensis 



MEGACYCLOTUS PONDEROSUS (Pfeiffer) 



Plate 24, Figures 19-21 



1851. Cyclostoma ponderosum PFE3rFEB, Free. Zool. Sec. Loudon, 1851, p. 243. 



1852. Cyclophorus ponderosus Pfeiffer, Monographia pneumonopomorum viven- 



tium, vol. 1, p. 97. 



1853. Cyclostoma ponderosum PrEiFFER, Martini-Chemnitz Conchylien Cabinet, 



vol. 1, sect. 19, p. 257, pi. 35, figs. 12-14. 

 1880. Amphicyclotus ponderosus Fischek and Crosse, Mission scientifique au 



Mexique et dans I'Am^rique centrale, vol. 2, pi. 35, fig. 3. 

 1888. Amphicyclotus ponderosus Fischer and Crosse, iUd., vol. 2, pt. 7, p. 147. 

 1890. Cyclophorus (Amphicyclotus) ponderosus v. Martens, Biologia Centrali- 



Amerieana, p. 5. 

 1900. Cyclophorus (Amphicyclotus) ponderosus v. Martens, ibid., Suppl., p. 597. 



Shell rather large, depressed-helicoid, covered with a thin pale- 

 brown periostracum, which appears to have irregular darker radial 

 zones. The nucleus consists of a little more than one small, well- 

 rounded, smooth whorl. Postnuclear whorls are well rounded, 

 slightly jflattened toward the summit and marked by coarse, decidedly 

 retractively curved diagonal cords and fine incremental lines. The 

 spiral corcls are about as wide as the spaces that separate them and 

 are rendered rather irregular by the incremental lines. Suture mod- 

 erately impressed. Periphery well rounded. Base well rounded, 

 openly umbilicated, with the umbilicus almost one-third the diameter 

 of the shell. The base is marked like the spire, but the umbilical 

 wall has only the continuation of the incremental lines, which here 

 develop into irregular riblets. Aperture circular, drawn into a con- 

 spicuous angle at the posterior angle of the aperture. Peristome 

 simple, thin on the outer lip and a little thicker on the inner. Parietal 

 wall covered by a moderately thick callus. 



The specimen described and figured, U.S.N.M. No. 32068, was col- 

 lected by F. Sarg in Guatemala. It has 5.5 whorls and measures: 

 Height, 25.7 mm.; greater diameter, 40.7 mm.; lesser diameter, 30.3 

 mm. 



Several specimens from northern Guatemala, U.S.N.M. No. 162309, 

 collected by Godman, are much larger, one having 4 whorls and 

 measuring: Height, 27 mm.; greater diameter, 48.5 mm.; lesser 



