19 
shape and ribbing, and, possibly, suture line; compared with all the known 
Skidegate species of Zemistephanus, however, it has much thinner whorls. 
Zemistephanus richardsoni (Whiteaves) 
1876. Ammonites richardsoni Whiteaves, Geol. Surv., Canada, Mes. 
Foss., I, pt. 1, p. 32, PI. 5, figs. 1, 2. 
HOLOTYPE 
Diameter 
Height, whorl . 
104-0 
29-8 
53-0 
43-6 
13-14 
86-5 
33-8 
58-0 
73-0 
39 1 
75-0 
40-4 
62-5 
39- 3 
73-0 
40- 3 
Thickness, whorl 
Width, umbilicus 
Tubercles per whorl 
As only one side of the specimen is preserved, the measurements for 
thickness are only approximate. The inner whorls are cadaconic and 
latumbilicate and the w r alls of the umbilicus are very steeply inclined. 
The ventral area is broad and arched. The umbilical enlargement begins 
at a diameter of about 80 mm., and near the last septum, where the whorl 
suture passes outside the tubercles. About three-fourths of a whorl 
of living chamber preserved, but marks on the specimen show that it 
extended nearly an entire whorl at least. Anterior end of ultimate whorl 
contracted both in height and thickness. The primary ribs end in large 
conical tubercles. The primary ribs are of less relief on the ultimate whorl, 
but persist to the end. The ventral area of the ultimate whorl is covered 
with small costulse which are arched forward a little. 
plesiotype 
Plate IX, figures 1,2; Plate X, figure 2 
Diameter. 
119-5 
24-7 
113-0 
28-0 
91-0 
32-0 
77-5 
32-8 
71- 5 
33-3 
72- 4 
39-5 
58-0 
34-0 
72-5 
39-0 
Height, whorl 
Thickness, whorl 
Width, umbilicus 
46-2 
16 
43-5 
38-0 
37-3 
Tubercles per whorl 
The first measurement is taken at the anterior end of the ultimate 
whorl preserved and the last a little more than 1^ whorls back. 
The number of tubercles per whorl increases with growth. This 
specimen is larger than the holotype and the umbilical enlargement takes 
place at a somewhat later stage of growth, at a diameter of about 90 mm. 
near the last septum and not far from the beginning of the ultimate whorl. 
There is about three-fourths of a whorl of living chamber preserved, but 
marks on the specimen indicate that it was at least nearly a whorl. It is 
possible to follow the ventral ornament back farther in this specimen than 
in the holotype; on the posterior part of the penultimate whorl there are 
small secondary ribs or costae, arched forward a little and numbering about 
7 per each tubercle. On the anterior part of this whorl they are similar, 
but are obscured over most of the surface by imprint of the ornament of 
the dorsal surface of the ultimate whorl, which leaves an impress of closely 
approximate irregular costulse. On the ultimate whorl the ventral orna- 
ment is not well preserved; the core appears to be smooth, but may not 
be so; there are some coarse undulations on the anterior part. 
