CEPHALOPODA. 8038 
Cyrtoceras. | 
the former are more abrupt than in any other species of the genus here noticed. 
None of the specimens are very well preserved, but the best of them retains nearly 
all of the body-chamber and seven air-chambers. The dorsum is very broad and but 
slightly arched. The seven air-chambers occupy a length of 19 mm. on the venter 
and 10 mm. on the dorsum. The body-chamber is 23 mm. in length, 23.5 mm. in 
major diameter at the base, and 22 mm. in minor diameter. The great difference 
in the outer and inner curves gives the shell a decided ventricose aspect about the 
base of the body-chamber. 
The specimens here described are in very close agreement with those upon 
which Whitfield based his species Oncoceras brevicameratum* from the Trenton beds 
at Beloit, Wisconsin. This is especially noticeable in the subcircular form of the 
‘septum. This species is, however, much less ventricose on the body-chamber than 
those which we here regard as representing O. pandion. 
Formation and locality.— In the Trenton limestone at Janesville, Wisconsin, and in the vicinity of 
Cannon Falls, Minnesota. 
Museum Register, No. 8303. 
Family CYRTOCERATID&, 
Genus CYRTOCERAS, Goldfuss, 1832. 
Though fully alive to the fact that the multitude of middle and late Silurian 
and early Devonian species which have been referred to Cyrtoceras, must eventually 
prove to be an association of phyletic inequalities, we still feel constrained to 
employ the term for a considerable number of the species here under consideration. 
These forms have been studied with care by Hyatt and most, if not all of the 
species here discussed will probably take their places within the genera intro- 
duced by him, namely ; Melonoceras, Oonoceras, Cranoceras and Hremocerast, but it is 
difficult in many cases to employ these terms with precision. In this author’s 
work Cyrtoceras does not appear as one of the “Genera of Fossil Cephalo- 
poda,” but the type of this old genus, C. depresswm, is assumed as the type 
of Cranoceras.#{This type-species is a middle Devonian shell, occurring in those 
later faunas of the Paleozoic where such forms ususully lack any evidence of a 
swollen body-chamber, but are likely to possess extended and more completely 
coiled tubes than in the Silurian faunas. It is among these later forms that the 
distinction between the genera Cyrtoceras and Gyroceras‘becomes very obscure, 
while in the Silurian shells the presence of an inflated tube is common and the 
*Geology of Wisconsin, vol. iv, p. 234, pl. vii, fig. 2. 
+Proc. Boston Soc. Nat. Hist., vol. xxii, pp. 280—282. 
“See the remarks by James Hall upon the impossibility of referring a large number of Devonian species with accuracy 
toeither genus: Palwontology of New York. vol, v, pt. ii- 
