208 PSEUDOCERATITES OF THE CRETACEOUS. | 
the true placenta correctly defined. In some parts of his text he also 
speaks of this as ‘‘ Placenticeras intercalare,” so that he is the sole authority 
for the combined names as used in this description. 
The ventral lobe as figured by Meek is very distinct from that of P. 
whitfieldi, as are also the entire sutures. This lobe is narrower, the 
branches are of the syrtale type, the siphonal saddle is more elevated, nar- 
rower, and is subdivided by a median marginal lobe with a minute bifid 
saddle; but these distinctions, except in a general sense, do not hold, since 
there are the same types of siphonal saddles in whitfieldi. The drawings 
by Meek, when compared with the originals, are accurate. The specimen 
on plate 23 is in the ephebic stage. The sutures overlap as much as in 
whitfieldi. A specimen having external shell in part preserved, from near 
Black Hills, South Dakota (purchased from Professor Ward), is 208 mm. in 
diameter. The last part of outer volution from lines of involution to venter 
is 112 mm., the first of the same being 50 mm. The greatest transverse 
diameter of the volution when it is 95 mm. is 44 mm., and when 50 mm. it 
‘is 29 mm. The volution is somewhat stouter than in whitfieldi and the 
involution somewhat less. The venter is a little broader and is bordered 
by two rows of tubercles of good size. These tubercles quite suddenly 
show decrease in size, and become much nearer at the same time on the 
last of third and on fourth quarter of this volution. They are present on 
both cast and shell. On the last part of outer volution they are almost 
obsolescent. On the first part of the living chamber they are opposite, 
then become again alternate, and as they decrease in size are again opposite. 
The ridges on the venter are slight, except in one short space, where 
they first become opposite. Tbe shell has numerous bands of growth. 
The chevrons are particularly prominent on last part of this volution and 
run into and form several longitudinal ridges on the outer half of lateral 
zone, while the costee are represented only by very broad, hardly 
perceptible, folds. The tubercles of the middle line ‘are of good size and 
become obsolete on the last quarter, changing at the same time with 
ventral rows of tubercles. Large tubercles are present on the umbilical 
shoulders, which sensibly decrease at the same time and also recede 
gradually from the shoulders outwardly. These nodes are elongated, 
forming parts of the costz that are more perceptible in their neighbor- 
hood. The outlines of the ventral zone are not sinuous between tubercles. 
