218 PSEUDOCERATITES OF THE CRETACEOUS. 
and its flatness in the ephebic stage, which is similar to stantoni and 
placenta. There are, however, some specimens with smooth venter, as in 
whitfieldi. A specimen in the collection of Columbia University, New York 
City, No. 10622G, from Upper Missouri River, has a diameter, partly 
estimated, of 150 mm. The last volution, with living chamber, is 76 mm., 
partly estimated; the umbilicus is 22 mm., and opposite the same volution, 
from lin of involution to venter, is 52mm. The volution is not so flattened 
or compressed, having slight ventral tubercles and slightly more gibbous 
* sides than in whitfieldi, and volutions not so deeply involute, as is shown by 
the breadth of the umbilicus. The line of involution is outside the line ot 
internal tubercles, and this more open aspect of the umbilicus and the solid 
aspect of the saddles and forms of the lobes agree more closely with the 
smaller form of placenta figured by Whitfield and are quite distinct from 
true whitfieldi of the same size. 
The living chamber is incomplete, but occupies about half a volution 
in length, and the aperture follows the bands of growth. The siphonal 
saddle has several minute denticulations even at this early age Sutures at 
diameter of 26 mm., from line of involution to venter, showed distinctly the 
syrtale type of outline which occurs in whitfieldi, only at a much earlier 
stage, short, broad saddles and corresponding lobes, with well-separated 
outlines and no appearance of bands free of sutures on either side of venter. 
No. 8238, U.S. National Museum, from Cow Island, Upper Missouri 
River, Fort Pierre or Fox Hills Group, is a very interesting fragment of 
this form. It is a good-sized fragment, showing the inner and outer volu- 
tions, with broad venters and proportions stouter than in typical pseudo- 
placenta or whitfieldi. The tubercles on the venter are not large, but quite 
plain. They disappear on the last of the outer volution, while the inner 
line persist, and there is no median line. There is on the last part of the 
outer volution, in the beginning of the gerontic stage, a distinct elevation 
along the line usually occupied by the median lateral tubercles, which I 
have never seen in whitfieldi, and which usually occurs only in syrtale, 
intercalare, and the more heavily tuberculated forms. This is also apparent 
in the full ephebic stage, but is less marked. The sutures are very similar 
and, in fact, not distinguishable from those of whitfeldi in some varieties. 
I have, in consequence, placed this shell under name of pseudoplacenta, 
although in general aspect it really seems to agree better with stantoni. 
