556 
EUTREPHOCERAS, Dekayi. 
Nautitus, Dekayi. Morton (Synop. Org. Rem., Pl. viii, Fig. 4). 
Loc., Dakotah, Cretaceous. 
Pl. xin, Figs. 4-8; Pl. 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 
cecum 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 cecum 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 metanepionie 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 
*TIt can be observed in the apex of Nautilus pompilius through the thin shell of the 
ananepionic substage. 
