497 
Figs. 16-19 give the adult and general aspect, and Fig. 32, 
Pl. x, shows the beginning of the contact furrow. ‘This begins 
only after contact with the apex and in strict correlation with the 
rounded immature form of the metanepionic and the temnocheilan 
or tetragonal trapezoidal form assumed by the paranepionic volu- 
tion. The sutures have ventral and dorsal saddles throughout the 
metanepionic, but in the paranepionic become straighter on the 
venter or with a faint lobe, and a similar change takes place in the 
dorsal sutures. The dorsal lobe is more easily perceptible after 
contact and becomes deeper with the increase in depth and breadth 
of the contact furrow and seems to be correlated in development 
with that modification, although it appears before this furrow is 
formed, 
Tainoceras. 
This genus has similar young to those of Metacoceras and proba- 
bly has a similar history. 
Centroceras. 
This genus, described in Genera of Fossil Cephalopods, possesses 
a typically quadragonal whorl in the adult stage of the less involute 
forms, but has a digonal whorl in the nepionic stage, and this 
becomes similar to that of Temnocheilus, that is trapezoidal in out- 
line and furnished with tubercles in the neanic stage. The 
contact furrow is faintly marked in some forms like Centroceras 
(Cyrt.) tetragonum, sp. Vern.,* but it is a mere narrow band on the 
dorsum. 
It is obvious in this genus that the impressed zone exists only in 
the later stages and after the whorls come into contact. Hall 
describes a slight impressed zone in Centroceras (Discites) ammontis, 
sp. Hally and shows the living chamber complete and nearly one- 
half of a volution in length. 
It is likely that Hall’s Gyroceras Stebos{ may prove to belong to 
genus Centroceras (Gyroceras) Ohioense, sp. Meek,§ is a very large 
shell of the Corniferous fauna of Ohio which has the form of whorl 
and single outer row of tubercles of this genus. 
* Trans. Geol. Soc. London, vi, Pl. xxx. 
Treble YeuvasUppl, et.al Pl. exxve 
Da len Cxexvils 
2 Pal. of Ohio, i, p. 230, Pl. xxii. 
PROC. AMER. PHILOS. SOC, Xxx1I. 148. 3K. PRINTED JULY 9, 1894. 
