198 PSEUDOCERATITES OF THE CRETACEOUS. 
species by the compressed and deeply involute volutions of the neanic 
stage, and these acquired first the flattened venter and helmet-shaped 
section:and then, as the ventro-dorsal diameters lengthened, the hollow 
venter bordered by smooth ridges and general aspect of Protengonoceras. 
The facts were ascertained by excavation as well as by studying the section. 
Unluckily the sutures were nowhere exposed in these inner volutions. 
The auxiliaries were visible later in the neanic stage after the volutions 
had become more compressed. They were then of the syrtale type, but 
their simple outlines showed that in the preceding Protengonoceras age they 
must have been very simple in outline and perhaps similar to those of 
Engonoceras. The nodes on the cast did not begin to appear on the 
umbilical shoulders until the shell was about 35 to 40 mm. in diameter and 
had entered upon the ephebie stage. The outer row of spines were not 
visible until later, and the age at which they appeared, except that it was 
later than the neanie stage, could not be ascertained. The ephebic stage 
has a stout volution with gibbous sides with proportions entirely different 
from those of the gerontic stage. At diameter of 26 mm. from line of 
involution to venter the transverse diameter at umbilical shoulders is 15 mm. 
and at 6 mm. distant from the venter the transverse diameter is 11mm. ‘The 
lateral zones are nearly flat and only slightly convergent and then converge 
rapidly but convexly to the venter which is broad, being here 5 mm. 
The last part of the neanic stage is 11.5 by 5 mm. at the unbilical 
shoulders and the convergence of the faintly convex lateral zones outwardly 
is constant to the venter, which is 1.5 mm. in breadth. The gerontic volu- 
tion on same section is 45.5 by 35.5 mm. at the umbilical shoulders and 
between tubercles; the plano convex venter is 27.5 mm., also between 
tubercles. The ventral line of tubercles and the concave area or ventral 
zone disappears in the gerontic stage, and the last measurements were taken 
after their disappearance near the basal sutures of the living chamber. 
Roemer’s figures are excellent, but they show a specimen much larger 
than mine, and but just entering the anagerontic substage. Roemer esti- 
mated that his shell, when complete, must have been a foot im diameter. 
The living chamber of Roemer’s specimen was broken away, or it 
would have shown similar gerontic characters. There is a centran, trace on 
the venter of Roemer’s figure which is present also on the venter of the 
ephebic stage in my cast. The trace is double, consisting of a faint depres- 
