﻿497 



Figs. 16-19 §i ye the a dult 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~) ammonis, 

 sp. Hallf 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) Ohioensc, 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, PI. xxx. 

 t Pal. N. Y., v, Suppl., Pt. ii, PL cxxv. 

 t Ibid., PI. cxxvi. 

 g Pal. of Ohio, i, p. 230, PI. xxii. 



