﻿495 



depressed oval, as shown in Barrande's figures. Fig. 17 gives a 

 cut farther in towards the narrower part of the umbilical perforation 

 and shows the paranepionic substage younger than in Fig. 16, and 

 with more depressed and approximately digonal outline. Above 

 this the paranepionic whorl is older than in Fig. 16, and with a 

 more decided impressed zone and broader transverse diameter ap- 

 proximating to the nephritic shape. 



In Fig. 18 the cut has passed beyond the perforation and shows 

 the paranepionic volution above when it first touches the dorsum of 

 the metanepionic or ananepionic substage below. The latter is 

 distorted because the cut goes through the inner or dorsal side of the 

 curve of the metanepionic and ananepionic substages. The oval in 

 the centre is apparently due to a cut through the fundus of the first 

 septum, which must be deeply concave. In Fig. 19 the cut has 

 approached nearer the ventral side of the apical chamber and is 

 apparently wholly within this and shows the increase in depth of 

 the impressed zone as the ananeanic substage begins and also the 

 decidedly nephritic outline which this at once assumes. This also 

 shows that the digonal outline of the volution below the centre 

 belongs to the neanic stage. In Fig. 20 the cut has passed close to 

 the outer side of the ananepionic substage and as in the centre it does 

 not intersect any septum it is probably wholly within the apical 

 chamber. This chamber must be very deep, as it is in Hercoceras 

 and some other forms. The broader shaded outline of the ananep- 

 ionic substage is the shell which is cut obliquely by the section. 

 The sections of the ananeanic whorl above and the metaneanic 

 below intersect a number of septa and show the passage to the 

 farther side of the umbilical perforation from that with which the 

 series began in Fig. 16. 



Temnocheilus. 



This genus is very similar in its general aspect to Hercoceras and 

 Anomaloceras, but it has distinct young and this shows that it has 

 been directly evolved from a cyrtoceran form and not from either of 

 these nautilian genera. 



The form known as Gyroceras proximum, sp. Barrande, PI. ciii, 

 has the tuberculations on the lateral angles, a trapezoidal whorl, 

 the siphuncle subventran and sutures and impressed zone as in this 

 genus, but until it is better known it is not practicable to decide 

 whether it belongs to this genus or to Hercoceras. 



