188 PSEUDOCERATITES OF THE CRETACEOUS. 
PLACENTICERATID& Hyatt. 
The young are similar to the adults of Engonoceratidee, having con- 
cave venters bordered usually by continuous ridges with compressed volu- 
tions, the lateral zones converging outwardly. In later stages of genera 
the venters are either concave, flat, or rounded, but there is no true keel 
present, although the venter may become acute in some species at a late 
stage of growth. The volutions in the earlier stages subsequent to the 
nepionic are apt to be more or less compressed, the venters assuming early 
the characters described above, any further sharpening of the venter occur- 
ring in later stages. The principal lateral saddles are, so far as known, bifid 
in the young and show in most groups a tendency to become more or less 
trifid in later stages. The multiplication of inflections of the suture lines 
increases with the involution of the whorls by growth and they become 
very numerous in some genera. 
PLACENTICERAS Meek. 
The species of this genus could be readily distinguished if it were not 
for the great range of form in the gerontic stage, which occurs in dwarfed 
as well as in large specimens, and is continually mistaken for the ephebic 
stage. 
The neanic shell is smooth, compressed, with at first a flat and then a 
concave venter bordered by smooth, entire ridges on the shell and on the 
casts, but has a stouter volution than later stages. It is, in other words, like 
the ephebic stage of Protengonoceras in external characters, but the sutures 
are of the Placenticeran type. Subsequently the ridges become tubercu- 
lated, the venter becomes much narrower and the sides also tuberculated and 
the volutions more compressed. These spines, nodes on the casts, are in 
three rows, but may be completely absent in some shells, as they are also 
in some old shells. In old age the venter again becomes flattened and 
smooth, and finally broader and rounded. 
The species are all connected so closely by intermediate forms that 
distinct lines are difficult to draw between contiguous species. 
The compressed and highly involute young show that those species, 
like P. guadulupe, having depressed volutions with broad venters, are 
senile forms in the phylum, or what I have named phylogerontic. ‘They 
