AMERICAN PALEOZOIC CEPHALOPODS 
259 
the casts of the interior of Amphicytoceras laterals, and a dif- 
ference in the vertical outline of the upper part of the ventral 
side of the living chamber, appear characteristic. 
48. AMPHICYRTOCERAS TANTALUM SP. NOV. 
PLATE XXIX, FIGS. 2 A, B, C 
Conch small, with its maximum lateral diameter, 27.5 mm., 
along the second camera beneath the living chamber. Above 
this point the lateral diameter shortens, at first slowly and then 
more rapidly for a height of 15 mm., where its lateral diameter 
is 19 mm. Above this the conch rises at least 8 mm. as a narrow 
neck, terminating in the specimen at hand at about the level of 
the aperture. The rate of enlargement of the lower part of the 
phragmacone is rapid. The conch is strongly depressed dorso- 
ventrally, the ratio of the dorso-ventral to the lateral diameter 
being as 24 to 27.5 mm. at the widest part of the conch. At the 
aperture the corresponding ratio is as 15.5 to 19 mm. At the 
base of the specimen it is as 14 to 16 mm. A little over 8 camerae 
occur in a length equal to the maximum width of the conch. The 
sutures are nearly directly transverse, but sag down a little on 
the ventral side of the conch. The septa are very moderately 
concave, and the siphuncle appears to occur one-third of the 
dorso-ventral diameter from the ventral side. Its passage 
through the septum at the base of the specimen is small, and 
the form and structure of its segments are unknown. 
Locality and horizon. — From the strata regarded as approx- 
imately equivalent to the Cedarville dolomite, at the abandoned 
railroad quarry in the northeastern part of Hillsboro, Ohio. 
Found by Henry Pavey. 
Remarks. — Amphicyrtoceras tantalum is characterized by its 
small size, the rapid constriction of the living chamber, and the 
elongation of the upper part of this chamber into a neck. 
MONOCYRTOCERAS GEN. NOV. 
Genotype: Monocyrtoceras lentidilatatum Foerste 
Conch related to Amphicyrtoceras, but enlarging at a much 
smaller rate; there is no dorsal gibbosity; and the siphuncle is 
much nearer the center of the conch, being only a short distance 
ventrad of the latter. 
