94 



NEW YORK STATE MUSEUM 



Septa transverse, closely arranged, those of the ephebic stage being 

 about 3 mm apart on the ventral, 2 mm on the dorsal side ; shallow, with a 

 low saddle on the venter. Siphuncle ventral near the margin (propio- 

 ventran Hyatt), tubular in the nepionic part, becoming slightly nummuloidal 

 in the following stages. Surface with fine concentric lines which are recurved 

 on the venter, where they are crossed by obscure longitudinal ridges. 



The species can not have attained a large size, for, among all the 

 specimens, none has been found to have a diameter of the living chamber 

 above 30 mm, this being the measurement of the largest specimen observed 

 which possesses distinct gerontic characters in the shallow later camerae. 

 Numerous specimens of average size with but two thirds of this measure- 

 ment, show the same gerontic features, and should therefore be regarded as 

 representing the normal size. An entire specimen whose living chamber 

 has a diameter of 18 mm, measures 54 mm along the ventral side. 



Horizon and locality. Rare in the Guelph horizon at Rochester, but 

 very common in the lower Shelby dolomite ; one doubtful specimen has 

 been taken from the upper Shelby horizon. 



Observations. The large representation of this species has allowed a 

 full elucidation of its characters ; and it is also possible to attempt a 

 generic reference more exact than to Cyrtoceras in its broad and common 

 application. The genus Cyrtoceras, as restricted by Hyatt, while including 

 similar exogastric, breviconic cyrtoceracones, with flat dorsum and elevate 

 venter, is defined as having the aperture contracted to a T-shaped opening, 

 and a large, nummuloidal siphuncle. Thus restricted, the generic term is 

 only applicable to Devonic forms, on which it was originally based, but we 

 have not been able to apply the term in this meaning because of lack of 

 evidence. Hence our use of the name in the foregoing is to be regarded 

 in a broad sense as covering species not at present referable with greater 

 exactitude. The genus Oncoceras, which is suggested by the short, 

 abruptly terminated septal part and the contracted aperture, is restricted 

 to laterally compressed forms of the Lower Siluric, with distinctly nummu- 

 loidal siphuncle. It is however clear that this genus is a primitive repre- 



