PLACENTICERATID &®. 229 
straight, and on the edges of the ventral zone the bases are entire and 
rounded. The first and second lateral lobes and the first, second, and third 
lateral saddles appear as inflections on the inner sides of the broad 
nepionic first lateral saddles. The third lateral lobes occupy the positions 
and are obviously the direct local representatives of the primitive primor- 
dialian lateral lobes. A large saddle, the primitive magnosellarian saddle, 
occupies the inner part of the sides and two minute marginal lobes and 
saddles are apparent on this. The first marginal saddle, the forerunner 
of the fourth and fifth saddles, is flat on the base and beginning to show 
an initial median marginal lobe; the future fourth lateral lobe, the inner 
saddle, has similar form but is still entire. This and the other specimens 
show that the great length of the third lateral lobes and the apical bend in 
the sutures and septa of adults are due to the retention of nepionic characters 
and that the great complication in the details of the outlines and the large 
number of lobes and saddles are based upon primitive nepionic outlines. 
This is also apparent in the internal double curvature of the septa, which 
are concave along the center and convex like those of most Ammonitinze 
only at the dorsal and ventral lobes. 
A fine young specimen, No. 18936, U. 8S. National Museum, Upper 
Missouri, enables me to add the following: Whole diameter is 55 mm., 
and the fourth volution is about completed. On the early part of this 
volution the sutures are the same as in the young specimen above 
described. The characteristic deeply cut saddles and lobes of the western 
form are already beginning to appear? and the first, second, and third 
lateral lobes have about the same proportions as in the adult, but the fourth 
lateral is only about half as long as the third lateral lobe. The sutures in 
this specimen are not so distinctly separated as in the young one at same 
age referred to, and are almost as closely intermingled as in the adult. 
The usual band free of sutures occurs on each side of the venter. On 
breaking down this specimen the sutures on the last quarter of the third 
volution were found to be more distinctly separated, the margins becoming 
simpler, but the peculiar bands free of sutures on either side of the venter 
are still present, and the species could hardly be mistaken even at this 
«The external shell is preserved in the umbilicus, but there are no tubercles, and yenter is 
smooth on the cast of this part. The outer layer of shell of the last quarter of fourth volution is 
preserved and shows same markings as above described at about same age in young specimen from 
Nebraska. 
