240 
AUG. F. FOERSTE 
Miller may belong here, though its dorsal outline is only faintly 
convex. 
35. ONCOCERAS CONSTRICTUM HALL 
PLATE XXXIX, FIGS. 2A, B, 3, 4 
Oncoceras constrictum Hall, Pal. New York, vol. 1, 1847, p. 197, pi. 41, 
figs. 6 a-f, 7 a-d 
Type . — Among the specimens figured by Hall that represented 
by figures 6 a, b, c may be selected as the type, not only because 
it is the first in the series of illustrations, but also because it is 
the largest, and evidently is a fairly typical example of the species 
as noiv knoivn. 
Conch curved lengthwise, slightly compressed laterally, and 
distinctly gibbous at the base of the living chamber and the top 
of the phragmacone, the gibbosity being conspicuous dorsally as 
well as laterally and ventrally. The radius of convex curvature 
of the vertical ventral outline is 30 mm. for the greater part of 
the length of the specimen, but at its apical end the rate of curva- 
ture is slightly greater. The dorsal side has a general concave 
curvature with a radius of 45 mm., but for a distance of 8 or 9 
mm. both above and below the base of the living chamber the 
dorsal outline changes to slightly convex, producing a conspicu- 
ous gibbosity here. Toward the apical end of the specimen the 
rate of concave curvature increases to a radius of less than 
20 mm. 
The conch is compressed laterally. At the base of the living 
chamber, where the dorso- ventral diameter is 24 mm., the lateral 
diameter is 20 mm. Toward the aperture the conch contracts 
not only dorso-ventrally but also laterally. At the aperture the 
dorso-ventral diameter is 18 mm. Since the living chamber of 
the specimen here described is crushed laterally, it is not suitable 
for showing the rate of lateral contraction toward the aperture, 
but in better preserved specimens the rate of contraction later- 
ally is greater than that dorso-ventrally, resulting in a slightly 
oval outline of the aperture. The curvature of the conch in a 
transverse direction is even on the dorsal side of the conch, but 
tends to be faintly angular on the median part of the ventral 
side. 
The base of the living chamber is exposed by a fracture. The 
height of the living chamber is about 20 mm. That part of the 
phragmacone which remains attached to the living chamber is 
