FOSSILS OF THE NIAGARA GROUP. 409 



The figures referred to are the dorsal and lateral views of a fragment, 



j,^^ g with a transverse section. The accompanying 



outline, fig. 5, shows the form of the entire cast 



of the interior, as nearly as can be given with 



the materials in my possession. 



This species, in the external markings of the 

 cast, is quite similar to Trochoceras notum, and not 

 very unlike T. costatum ; but the enrollment of 

 the shell in the same plane is a distinguishing 

 feature. 



Formation and Locality. — In limestone of the 

 age of the Niagara group, at Bridgeport, Illinois. 

 /• 



CtRTOCERAS HERCULES, WiNCH. AND MaR. 



PLATE XVII, FIGS. 6, 7. 



Lituites hercules, Winchell & Makct; in Mem. Bost. Soc. N. H., I, p. 102, Plate iii, flg. 9. 

 Cyrtoceras (Phragmoceras ?) amplicorrte, Hall ; in 1st edit. Twentieth Rep. N. Y. State Cab. 

 N. H., p. 358. 1867. 



Shell large, strongly curved, making an entire volution, rapidly expand- 

 ing toward the outer chamber, which is less arcuate than the septate 

 portions of the shell; transverse section broadly elliptical, becoming 

 a little flattened on the sides towards the outer chamber. Septa 

 rather deeply concave and comparatively distant ; three of them, 

 measured on the dorsum, being nearly equal to the diameter of the 

 outer part of the largest one measured. The dorsal margins of 

 the septa are advanced so that a direct transverse section would cut 

 the ventral margin of the next preceding one. Siphuncle small, 

 central or subcentral. Entire form of aperture unknown. Surface 

 finely striated, with the striae curving backward to the dorsum. 



This is one of the most robust species of the genus known in our rocks, 

 and, were the outer chamber entire, would measure full seven and a half 

 inches in height, with a transverse measurement of more than five and 

 a half inches. The dorso-ventral diameter of the base of the chamber 

 of habitation is two inches and eight-tenths, with a transverse diameter 

 at the same point of two inches and three-tenths. It is likewise remark- 

 able for its great curvature and central siphuncle. 



The flattening of the sides gives the shell in some respects the aspect 

 of Phragmoceras ; but the curvature is so broad and regular, and the 



Cab. Nat. 52 



