232 PSEUDOCERATITES OF THE CRETACEOUS. 
begins with a slight transverse constriction, after which the transverse 
diameters increase considerably and the volution is stouter and larger. 
The largest specimen so far recorded is in the University Museum in 
New Haven (locality, Bad Lands, South Dakota). This has been meas- 
ured by Dr. C. E. Beecher, who also very courteously sent me a sketch and 
notes on its characteristics. The diameter of the entire fossil is 630 mm. 
(24% inches). The ventro-dorsal diameter of aperture from line of mvolu- 
tion to venter is 300 mm., and the transverse 150 mm., showing how much 
the volution has broadened in this extreme gerontic stage. The first 
quarter of outer volution has a flat venter 8 mm. wide, the edges rounded. 
The living chamber is perfect and occupies one-half of a volution, showing 
no decrease in length as compared with smaller specimens. There is, how- 
ever, the usual decrease in the amount of involution of the whole of the 
outer volution. The venter loses its flatness entirely on the second quarter 
of the outer volution and on central parts of living chamber, becomes 
rounded, and the sides then become gibbous at a short distance inside the 
venter, as is not uncommon in old age. There is a tendency to resume the 
form of syrtale and of its own earlier stages. Near the aperture the geron- 
tic flat zone noticed in other specimens returns upon the centran part of 
the venter, but the latter still remains rounded with the same gibbous out- 
line to the outer parts of the sides. The aperture, as described in other 
specimens, has a short, broad rostrum and low, broad lateral crests. 
Locality: Nebraska; South Dakota; Colorado. 
Age: Fort Pierre group, Upper Cretaceous. 
PLACENTICERAS WHITFIELDI variety TUBERCULATUM Hyatt. 
Pl. XLVH, fig. 5. 
I have so far seen only three shells from Bad Lands, South Dakota, 
having a yery faint line of median lateral tubercles both on cast and shell, 
as well as minute denticles on the sides of the venter and tubercles on the 
umbilical shoulders. The two outer rows disappear at the end of the neanic 
stage, when the shell is about 50 mm. in diameter. These are obviously 
transitional to P. pseudoplacenta, but have much finer tuberculations and 
more complicated sutures. 
Locality : Bad Lands, South Dakota. 
Age: Fort Pierre group, Upper Cretaceous. 
