NIAGAEA LIMESTONE OF CHICAGO, ILLINOIS. 99 



band, since the surface of the body whorl shows numerous transverse lines, curving re- 

 trally from the suture to this peripheral angle. The rapid increase of the whorls throws the 

 apex much nearer to one side than the other. 



Height, 30 inch; height of body whorl, .18 inch; diameter of base, .44 inch; apical 

 angle, 76. 



PLATYCERAS (Conrad) Hall. 



Platyceras campanulatum W. and M. 

 Plate II. figure 16. 



Whorls three or four, in contact ; spire scarcely elevated above the last volution ; last 

 volution greatly enlarged, with a nearly circular section, an angulated periphery and 

 trumpet-shaped, circular aperture. Columella none ; umbilicus minute ; lip not modified 

 by the penultimate whorl. Peripheral ridge in the middle of the whorl or a little above. 

 Surface marked by extremely faint longitudinal striae, and by remote, obscure, transverse 

 ridges, not retroflected in passing the periphery. 



This species seems to be the representative of Platyostoma niagarense Hall. It is inferior 

 in size to full-grown specimens of that species, and differs from it in the possession of an 

 umbilicus, and in the completeness of its peristome. It is to be observed, however, that 

 the entire peristome is possessed by many specimens from Waldron, Indiana, that have 

 been referred to Platyostoma niagarense ; and that the two genera run inseparably together. 

 Should we retain but one of these generic names, Platyceras has the prior claim. 



HOLOPEA Hall. 



Holopea niagarensis W. andM. 



Plate II. figure 18. 



Shell small, consisting of four or five very convex whorls in a moderately elevated spire 

 having an apical angle of 58 ; the last whorl more rapidly enlarged than the preceding, 

 rounded below, and apparently excavated into an umbilicus. Suture deeply impressed, 

 each whorl being almost completely above the level of the preceding one. No surface- 

 markings visible on casts. 



Height, .25. inch ; diameter of last whorl, .20 inch. 



Holopea chicagoensis W. and M. 

 Plate II. figure 18. 



Shell small, consisting of five or six moderately convex whorls in a sub-elevated spire, 

 having an apical angle of about 42 ; body whorl proportionally higher, but not more en- 

 larged than the preceding. The outer side of the whorls is somewhat flattened, becoming 

 rapidly incurved in the upper part, toward the suture ; whorls apparently overlapping. 

 Base of last whorl rounded into a minute umbilicus; transverse section of whorl nearly 

 circular. Surface of casts smooth. 



Height, .35 inch; diameter of last whorl, .25 inch. 



These two species of Holopea existing only in the form of casts it is impossible to 

 distinguish certainly from Cycloncma, though that genus is said to be always exumbilicate. 



