•360 
PALAEONTOLOGY OF NEW YORK. 
previously considered as undoubted Cyrtoceras, beginning with C. alternatum, 
C. eugenium and C citum, 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 XLVII, FIGS. 1, 2; CXIV, FIG. 11. 
Cyrtoceras metula, Hall. Descriptions of New Species of Fossils, etc., p. 44. 1861. 
“ “ “ Fifteenth Rep. N. Y. State Cab. Nat. Hist., p. 72, pi. 9, fig-. 7. 1862. 
“ “ “ Illustrations of Devonian Fossils : Cephalopoda, pi. 46, tigs 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 liabi- 
