576 
shifted less, and consequently the upper row of tubercles is brought 
over the trace made by this organ in the cast of the interior. 
Great irregularity appears in this substage, the costee may be bifur- 
cated at the tubercles and between them, or they may run across 
the venter and be bifurcated at the base of the tubercle of the oppo- 
site side. This ornamentation is similar to the fragment figured by 
Gabb in Paleontology of California as Ammonites Cooperi, Vol. i, 
Plesay: 
The dorsal crests formed by costz occupy the somewhat flattened 
dorsum of the early part of the paraneanic substage, but when the 
twisting begins these dorsal crests begin te be unsymmetrical also. 
In other words, the lines of growth and costz assume the usual 
direction and aspect of turbinate shells, whether Gasteropoda or 
Cephalopoda. 
The spiral is quite irregular in the anephebic substage, but is 
more regular in the metephebic and parephebic substages. In the 
parephebic substage the tubercles disappear in this specimen. 
There is a decided contraction of the transverse diameier of the 
spiral in this substage. 
The absence of the tubercles on this substage is similar to the 
change that takes place in JVostoceras nebrascense at the same age 
and enables one to classify all the substages satisfactorily. 
Exiteloceras,* n. g. 
After a careful survey of the forms referred to this genus, it 
has become evident that there is a series having the following char- 
acteristics and quite separable from the full-grown stages of any 
other genus referred to the Nostoceratide. They are, however, 
not so easily separated from the young of Nostoceras if my obser- 
vations are correct, since the single costz with two lines of tuber- 
cles are found in the young of that genus and of Emperoceras. 
This, however, is entirely in accord with the system advocated in 
this and other publications and is in my opinion another argument 
in favor of distinguishing the group by another name. 
The series of forms figured by Meek in his /zvertebrate Paleon- 
tology, Pl. xxi, have a single costz with two rows of tubercles, each 
costation being tuberculated. ‘The ephebic stage is helicoceran 
and the gerontic stage probably has the retroversal living chamber. 
“E=izyihog, becoming extinct. 
