AMERICAN PALEOZOIC CEPHALOPODS 
267 
strengthening the margin of the aperture. Along the ventral 
side this margin appears to curve down\vard, forming a distinct 
hyponomic sinus. 
The upper part of the phragmacone narrows so rapidly in an 
apicad direction that it is probable that the phragmacone was 
very short. The sutures of the septa rise distinctly from the 
ventral toward the dorsal side of the conch, and the height of 
the camerae enlarges in this direction. The siphuncle is exposed 
close to the ventral side of the conch. Within the lowest camera 
its diameter was at least 14 mm. The location of the passage of 
the siphuncle through the septum evidently was similar to that 
in Diestoceras indianense. 
54. CYRTOGOMPHOCERAS GEN. NOV. 
Genotype: Oncoceras magnum Whiteaves. Trans. Royal Soc. 
Canada, vol. 7, sec. 4, 1890, p. 79, pi. 15, fig. 1 
Conch distinctly curved lengthwise, endogastric, and laterally 
compressed. The dorsal side is strongly convex, and the ven- 
tral outline of all but the uppermost part of the phragmacone is 
slightly concave, but along the top of the phragmacone and the 
adjacent part of the living chamber the ventral outline is dis- 
tinctly gibbous. Conch compressed laterally. Sutures of the 
septa rising strongly from the ventral toward the dorsal side of 
the conch along the upper part of the phragmacone. Aperture 
correspondingly oblique, the living chamber being strongly con- 
tracted toward the latter. Siphuncle near the ventral wall of 
the conch, its segments strongly nummuloidal. 
In addition to the genotype, Oncoceras magnum Whiteaves, 
this genus includes also Oncoceras ivhiteavesii Miller, a species 
originally described by Whiteaves as Oncoceras gibbosum. 
Oncoceras intermedium Whiteaves is similar in having a 
strongly nummuloidal siphuncle near the concave or ventral side 
of the conch, and in having the septa rise strongly toward the 
dorsal side, but the conch is more slender, there is nO' gibbosity 
along the ventral side of the conch, and there is no evidence at 
present of the living chamber having a structure similar to that 
of typical Cijrtogomphoceras, 
