250 
AUG. F. FOERSTE 
those figured by Whitfield. Two of these retain enough of the 
living chamber and of the attached part of the phragmacone to be 
referred with confidence to Oncoceras pleheium. One of these has 
a dorso-ventral diameter of 17 mm. at the base of the living 
chamber, the other of 19.5 mm. A third specimen consists of a 
body chamber, expanding toward the aperture as in Cyrtorizo- 
ceras minneapolis (Clarke) and Cyrtorizocems scofieldi (Clarke). 
Not enough remains of the other two specimens to make their 
relationship to Oncoceras pleheium certain. 
Locality and horizon . — Described from the “Buff limestone o-f 
the Trenton limestone group, at Beloit,” Wisconsin. According 
to Bassler’s Bibliographic Index, the horizon of this species is 
the Platteville member of the Black River. 
Types, No. 996, American Museum of Natural History, in 
New York City. 
Margin of the aperture. — One of the specimens in the Ameri- 
can Museum of Natural History apparently presents features 
both of the exterior and of the interior of the upper part of the 
shell, but its chief interest lies in the fact that it retains a sharp 
outline of the margin of the aperture. This margin is most 
elevated along the middle of the lateral sides, and from this point 
it slopes slightly downward both toward the dorsal and toward 
the ventral side; however, the slope toward the ventral side of 
the conch is slightly greater than that toward the dorsal side, 
compared with the direction of the central axis of the living 
chamber. The hyponomic sinus evidently is very shallow. When 
viewed from above, however, the aperture contracted distinctly 
ventrad of its center, as in the type of Oncoceras pleheium. 
Along the dorsal half of the specimen, the living chamber is 
contracted slightly but distinctly at a level about 5 or 6 mm. be- 
low the margin of the aperture. Toward the ventral side this 
contraction is scarcely 4 mm. from the margin of the aperture. 
Above this point of contraction the specimen retains about the 
same dimensions as far as the aperture. Since no trace of the 
septa is shown by the specimen, this contraction appears to be 
a feature of the exterior surface of the shell as well as of its 
interior, but ordinarily it would be regarded as characteristic of 
the interior of the living chamber alone, due to a thickening of 
the interior of the shell along the line indicated. 
Judging from the contours of the specimen, the height of the 
