56Q PALAEONTOLOGY OF NEW YORK. 



previously considered as undoubted Cvrtoceras, beginning with C. alternatum, 

 C. eugeniiim and C cilum, together with Gyroceras Nereus, G. trivolve, G. laciniosum, 

 G. Matheri, G. paucinodum and G. undulatum, constitute a seiies of similar forms 

 with a regular gradation of form and curvature, from a nearly straight tube to 

 one with several closely coiled volutions. A second group of stouter forms, 

 beginning with Cyrtoceras Jason, and ending with Gyroceras Cyclops, present a 

 similar gradation of curvature. That the genus Gyroceras or Cyrtoceras must 

 include such forms as Cyrtoceras (Gyroceras) eugenium and the following species, 

 is evident from the close relations existing between them, and the regular 

 gradations of form and curvature, reaching the maximum in Gyroceras undula- 

 tum and G. Cyclops. Any generic separation of these forms, based on the 

 amount of curvature of the tube, overlooks the features which constitute the 

 more prominent characters, and unite the species in a single natural group. 



In the Silurian formations the reference of the species to Cyrtoceras is 

 easily made ; but when the forms considered as Gyroceras are gradually devel- 

 oped in the Devonian formations, the limits of the genera are not clearly 

 defined. If the latter genus is to be recognized in this country, it must 

 certainly include such forms as Gyroceras Cyclops and G. undulatum, which are 

 not generically removed from Cyrtoceras Jason and C. eugenium. 



In the notice which is intended to supplement this volume an attempt will 

 be made to furnish some satisfactory generic distinctions, and to arrange the 

 species with regard to their natural dependence and interrelations. 



Cyrtoceras (Gomphoceras) metula. 



PLATES XL.VII, FIGS. 1, 2; CXIV, FIG. 11. 



Cyrtocertu metula. Hall. Descriptions of New Species of Fossils, etc., p. 44. 1861. 



" " Fifteenth Rep. N. Y. State Cub. Nat. Hist., p. 72, pi. 9, fig. 7. 1863. 



" " " Illustrations of Devonian Fossils : Cephalopoda, pi. 46, figs 1,2. 1876. 



Shell slender, arcuate, exogastric. Transverse section elliptical, with the 

 diameters in the ratio of three to four. Longitudinal section sublimate. 

 Plane of greatest transverse section near the middle of the chamber of habi- 



