PLACENTICERATIDZ. 189 
are not seaphitoid, as stated by Munier Chalmas, except in the sense that 
Scaphites is an extreme case of the same tendency to reproduce senile char- 
acters early in the ontogeny and to such an extent that the ephebic form 
becomes more or less influenced by them. The species form a series, there- 
fore, leading off from shells, like P. syrtale in one direction into P. guada- 
lupe and in the other into whitfieldi. The intermediate modifications 
connecting this genus with Protengonoceras are probably partly represented 
by Diplacmoceras, but this genus is not a primitive, although it seems to be 
a phyloneanic form. 
The solution of the species problem appears to lie principally in the 
development of tubercles and the correlative stoutness of the volutions. 
Shells having heavy tubercles in three lines usually also have sutural pecul- 
larities that enable one to distinguish them. The peculiarities of the gerontic 
stage are also distinct, as will be noticed in the descriptions. In some spe- 
cies the neanic stages are quite distinct in their sutures, although alike in 
their forms at the same age, and perhaps with more materials the study 
of these and the development of sutures may give good results. The 
most highly modified form as compared with its own neanic stage is 
certainly guadalupe, but although the sutures are complicated in outline, 
they are not so complex as those of whitfieldi, and the modifications of 
form are distinctly in a phylogerontic direction. The varieties of each 
species and the development poiut to the most prevalent syrtaloid form 
as presenting more than any other purely progressive characters. This 
form has moderately compressed involute whorls, with three lines of nodes, 
narrow venter, and steadily complicating sutures. The same variations in 
the species and in the individual point consequently in one direction toward 
guadalupe and in another toward whitfieldi. This last is reached through 
species like stantoni and pseudoplacenta, 11 which the median lines of 
tubercles become permanently obsolete and the outer and inner lines 
become less prominent and in many specimens of whitfieldi are absent, 
The interesting fact in this connection is that whitfieldi, which, as compared 
with its own young, is the least modified of all the forms, has the largest 
shells, the least affected by gerontic degeneration, and at all stages the 
most complex sutures. 
These facts also show in a marked way the law of retardation of 
development. This was joined by Cope with acceleration, but so far as 
