102 NEW YORK STATE MUSEUM 



with these ridges and covering both ridges and the intervening sulci, are 

 finer lines which are crossed and cancelated on the early shell by revolving 

 lines of the same size. With the obsolescence of the ridges, the concentric 

 lines retained to the aperture become more and more conspicuous and 

 defined. The revolving lines may also be seen under favorable conditions 

 in later growth. 



The living chamber continues in the curve of the volutions, though, 

 in a gerontic stage, it continues in a nearly direct line. The aperture is 

 large, provided with a shallow dorsal and probably a deep ventral sinus. 



Localities. Not infrequent in the lower Shelby dolomite and at 

 Rochester ; rare in the upper Shelby bed. 



This form was first described by McChesney from the " Niagara divi- 

 sion " of Joliet and on the Kankakee river, 111. Professor Hall obtained his 

 specimens from Racine Wis. and Whiteaves reports two specimens from 

 the Guelph at Hespeler. The shell is definitely dextral and is thereby 

 distinguished from the otherwise closely allied form termed T r o c h. 

 aeneas Hall 1 from the dolomites at Lyons la. 



The generic term Trochoceras was employed by Barrande in 1847 and 

 independently introduced by Hall in 1852. The former's conception of the 

 generic value is expressed in the species T. optatum Barr. (Etage E), 

 which is very closely allied to T. desplainense, but is a sinistral shell. 

 Hall's type is T. gebhardi Hall of the Coralline limestone, a shell with 

 high spire, smooth whorls and of quite distinct aspect from the group under 

 consideration ; it has been made by Hyatt the type of his genus Mitro- 

 ceras. Hyatt has taken Barrande's Troch. optatum as the type of his 

 genus Sphyradoceras, and we have therefore employed Trochoceras here in 

 the restricted sense ascribed to Sphyradoceras by its author. 



Hyatt considers Sphyradoceras as an offshoot of Spyroceras, a genus 

 with, in early growth, longitudinally ridged, and, in later, annulated longi- 

 cones ; and mentions the presence of longitudinal ridges among the gene- 

 ric characters of Sphyradoceras. Our material does not show this last 



1 N. Y. State Cab. of Nat. Hist. 20th An. Rep't, revised ed. pi. 25, fig. 16. 



