40 
PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 
except on young specimens of 8 to io mm. or less, when they continue 
across the venter; ribs in part simple, part branching once just before 
reaching the middle of the flank and part consisting of short ribs on the 
outer half of the whorl interpolated between the long ones. 
Sutures having the same general characteristics as in the preceding 
species, with broad, not very deeply dissected, lobes and saddles. The 
forms and proportions of the several lobes and saddles are nearly the 
same in the two species, the principal difference being in the more com- 
plicated and deeper dissection of the suture in H. patagonense , but this 
difference is not as great as a comparison of the figures would indicate, 
because both the sutures of H. argentinense are taken from much smaller 
and less mature specimens than that of the other species, and the larger 
one is considerably weathered. There is only one auxiliary lobe visible 
on the flank, and the dorsal portion of the suture, as seen on a specimen 
about 20 mm. in diameter, shows a slender, pointed and dentate, antisi- 
phonal lobe and two rather simple, paired dorsal saddles on either side. 
Some variations in sculpture at different stages of growth have already 
been indicated and, as the sculpture shows a decided tendency to become 
less prominent on the larger specimens which are septate throughout, it is 
possible that the last whorls of fully adult specimens become smooth as 
in H. patagonense. 
Young shells also differ greatly in form from the adult, the umbilicus 
being relatively larger (that is the shell less involute) and the whorls 
much more convex, so that in shells io mm. or le£s in diameter the breadth 
of the aperture is nearly or quite equal to the height, but these proportions 
change rapidly to those of the adult. The following measurements of 
four specimens indicate the normal proportions of the species and the 
change from youth to maturity. The measurements under I are from the 
largest specimen in the collection, which was too badly weathered for figur- 
ing; those under II and III from the specimens represented by figures 4 
and 5 on plate IX ; and those under IV from the inner whorls of a speci- 
men that was broken down for the study of the early stages. 
I 
II 
III 
IV 
Diameter 
128 
122 
48 
10 
Umbilicus 
28 
25 
1 1-5 
3 
Height of aperture 
68 
65 
23 
4.8 
Breadth of aperture 
37 
35 
H 
4.2 
