AMERICAN PALEOZOIC CEPHALOPODS 
243 
12 mm., and the dorso-ventral one is 14 mm. In addition, the 
aperture is narrowed by two low but distinct ventro-lateral sad- 
dles, which rise above the level of the dorsal outline of the 
aperture and converge more or less toward each other. In the 
vicinity of these saddles the dorso-ventral parts of the living 
chamber are more or less flattened. The general outline of the 
aperture is pear-shaped, with the narrow part on the ventral side. 
The upper part of the interior of the living chamber is dis- 
tinctly constricted by a thickening of the shell. Along the dorsal 
and lateral sides of the aperture this thickening begins 3 mm. 
beneath the margin ; along the ventral side it begins about 1 mm. 
beneath the aperture. 
Along the ventral side, 5 camerae occupy a total length of 
13 mm. The sutures of the septa rise slightly from the dorsal 
toward the ventral side of the conch. The sutures are nearly 
straight, their downward curvature along the lateral sides of the 
phragmacone being very faint. The general concavity of the 
septa is slight, equalling scarcely 1 mm. The siphuncle is about 
one-third of a millimeter from the ventral wall. Its lateral 
diameter, where passing through the septa, is 0.7 mm., widening 
to about 0.9 mm. within the camerae, the general form of its 
segments being only slightly fusiform. 
The shell is relatively thick, equalling nearly 1 mm. along the 
dorsal and dorso-lateral parts of the upper half of the living 
chamber, and nowhere thinning to less than half a millimeter. 
The transverse striae are merely lines of growth. Along the 
lower half of the living chamber these lines are almost directly 
transverse along the dorsal half of the circumference of the 
chamber ; but on the ventral side they curve distinctly downward 
as far as the median part of this side. However, toward the 
upper part of the living chamber, the transverse striae rise more 
and more strongly from the dorsal toward the ventro-lateral 
parts of the conch, outlining successive stages in the development 
of the ventro-lateral saddles, which gradually increase in promi- 
nence until they attain an elevation of 2.5 to 3 mm. above the 
general level attained by the aperture along its dorsal side. 
From the crest of these saddles the lateral margins of the hypo- 
nomic sinus slope diagonally downward as far as its ventral end 
which descends 1 mm. or more beneath the general level at- 
tained by the dorsal side of the aperture. The general form of 
