﻿556 



Eutrephoceras, Dekayi. 



Nautilus, Dekayi. Morton (Synop. Org. Rem., PI. viii, Fig. 4). 



Loc, Dakotah, Cretaceous. 



PI. xiii, Figs. 4-8; PI. xiv, Fig. 1. 



The ananepionic substage in this species is very obtuse and al- 

 most saucer shaped, the whorl increases so rapidly in all its diame- 

 ters. The cicatrix is present on one specimen and is a double 

 depression with a dividing ridge on the cast of the apical chamber. 

 There is a peculiar plate of nacreous matter which may be the 

 equivalent of a similar plate which fills in the apex of the shell in 

 Nautilus pompilius or it may be simply a remnant of the apical 

 deposit which has this peculiar form. However this may be, the 

 caecum is seen through it in one specimen, and in another it can 

 be seen in the same position, although the plate is not visible, the 

 apex being more completely covered by the external shell. 



It seems clear that the dark spots observed in these two speci- 

 mens were due to the presence of the caecum filled by a dark, 

 sparry deposit and showing through the nacreous layer.* If so 

 this organ is close against the venter of the apical chamber. I was 

 not able to see the youngest septa, but there are evidently very few 

 of them and the one shown in Fig. 4 is probably either the third 

 or fourth septum. 



The metanepionic substage is not so smooth as the ananepionic, 

 and although it is difficult to observe without making a section, I 

 am quite sure that there is a faint dorsal furrow present before the 

 gyroceran bend begins. The longitudinal ridges and the trans- 

 verse bands with the usual crenulated edges begin to be observable 

 in this substage. 



The bend which begins the paranepionic substage is very abrupt 

 and almost at right angles to the dorsum of the metanepionic sub- 

 stage and has a deep dorsal furrow. The umbilical perforation is 

 consequently so small and arcuate that it is very difficult to observe. 

 In Fig. 6 the lateral angle of the shell and of the first septum that 

 is built upon the dorsum of the apex has been cut off and shows the 

 opening of the umbilical perforation in part, but has a misleading 

 outline since it is just the reverse in shape of the true internal per- 

 foration. It, however, shows that there is a perforation as does 

 also Fig. 4. This shell must grow in these younger substages with 



*It can be observed in the apex of Nautilus pompilius through the thin shell of the 

 ananepionic substage. 



