162 PSEUDOCERATITES OF THE CRETACEOUS. 
ENGONOCERAS SERPENTINUM (Cragin). 
Pl. XIX, figs. 7-14; Pl. XX, figs. 1-5. 
Sphenodiscus belviderensis var. serpentinus Cragin, 1900, Colorado Coll. Studies, 
Vol. VII, p. 31, pl. 2, figs. 4-6. 
Three casts of this species (Loc. No. 1489) are well preserved in a 
matrix of hematite. The diameter of one (1489 b) is 58 mm., the volution 
is 29 mm., the umbilicus 7 mm.; the same volution opposite, measured 
from lines of involution to venter, 22 mm., no shell being present. 
The involution conveys the larger part of each outer whorl, but the 
internal volutions are plainly visible in the umbilicus and the involution is 
obviously much less in the younger stages than in adults. The volutions 
are much compressed. The venter is, however, flattened and slightly con- 
cave in the ephebic stage, and in the gerontic stage it becomes asymmetrical 
and sinuous through the development of the large tubercular terminations 
of the alternating costee. In the ephebic stage the costee are sigmoidal and 
only slightly developed; in the gerontic stage these become broad on the 
outer parts with an external and internal line of nodes, and become obscure 
internally or umbilicad of the second row of nodes, but they are obviously 
confluent to the internal or third line of smaller nodes on the umbilical 
shoulders. These nodes, like the costae, come in on the casts at a late 
ephebic substage, the side of the younger whorls being smooth. The 
tubercles of the outer line in the one specimen (1489 b) are sharper and 
the coste at the points convergent, whereas in the second specimen from 
the same locality the latter broaden out and the tubercular terminations 
are more elongated This, however, resembles those of the other specimen 
in the ephebic and anagerontic substage, the marked elongation coming in 
with the metagerontic substage. 
A similar disposition to broadening out of costz is also observed in the 
larger specimens in extreme age, but is not so marked and the venter also 
remains narrower. The living chamber is not complete, but it must have 
been about one-half of a volution in length on the line of involution. Other 
specimens show the same, but none give the outer margins. The imner 
lines of tubercles remain close to the umbilical shoulder, receding outwardly 
very slightly in extreme age. 
