238 PSEUDOCERATITES OF THE CRETACEOUS. 
specimen is certainly in the gerontic stage, with the median lateral row of 
tubercles already on the borders of the venter. The ventral rows shown 
in fig. 3 of pl. 5 have disappeared entirely, and the venter is smooth and 
flat and very broad, the tubercles nearly opposite. The inner row of nodes 
begins to recede from the line of involution very early and gradually 
approximates to the outer row. On the last quarter these tubercles again 
begin to approach the umbilical shoulders, increasing their distance from 
the outer line—a movement not paralleled in any American form I have 
as yet seen. 
Age: Lower Senonian (Santonian). 
PLACENTICERAS INCISUM n. sp. Hyatt. 
Placenticeras syrtale (pars) Grossouvre, 1893, Ammonites Craie supérieure, pl. 8, 
fig. 1 (no others). 
In this form the volution is described and figured as compressed and 
smooth until a late stage. The specimen is in the parephebic substage on 
the first quarter of the outer volution; i. e., the median lateral line of tubercles 
is close to the ventral line. The venter is, however, still concave, narrow, and 
tuberculated. On the second quarter the venter broadens to these lines of 
tubercles, the ventral ones disappear, the inner line of large nodes appears 
first at the center of the lateral zones and rapidly approximates to the outer 
lines, being joined to them by costee. There are only five of these nodes, 
and while the third and fourth are nearest to the venter, the fifth is set 
somewhat farther away inwardly, thus mdicating, as in grossouvrei, the 
same tendency of the nodes to return inwardly in extreme age. The 
peculiar broad, concave venter of the paragerontic substage is also akin to 
the aspect of the venter in an earlier substage in grossowvrei, but is a great 
exaggeration of this tendency, and occurs after the venter has become 
convex in the anagerontic substage, as is shown on the lower outline of 
fic. 1b. These figures are so fine that these observations are made upon 
their authority, and show a great difference between these and related 
American forms. 
Grossouvre’s reference of this variety to P. milleri of Hauer allows 
more latitude for variation than is given in this memoir. Milleri has only 
one row of tubercles, and the form of the venter is distinct. 
Age: Senonian. 
