196 PSEUDOCERATITES OF THE CRETACEOUS. 
with those which had already been passed through by the surrounding 
plates of the corona during the stages of the ontogeny. 
In other words, there is a local ontogeny in newly introduced lobes 
and saddles parallel with the developmental ontogeny of the same elements, 
and, in growing, each newly introduced internal lobe or saddle repeats 
locally the same stages of development. The arrest of development, which 
takes place habitually in all forms of Ammonitine, imternally stops the 
progress of each lobe and saddle at different stages in proportion to the 
distance of the lobe or saddle of the auxiliary series from the line of involu- 
tion. In genera like Engonoceras, however, the entire suture is arrested im 
development and presents therefore a certain resemblance to the young of 
other Ammonitinze in the neanic stage and especially to the young of 
Placenticeras after the three entire principal lobes and saddles and the entire 
lobes and saddles of the auxiliary series have appeared. It may be objected 
that the division between the species is too minute and artificial, and it is 
admitted that this objection is in one sense well founded. 
There is no real line between P. guadalupe, sancarlosense, and planun, 
nor between newberryi and guadalupe, nor between guadalupe, sancarlosense, 
syrtale, intercalare, and placenta, nor between intercalare, stantoni, pseudopla- 
centa, and whitfieldi. As a matter of fact there is no real break, such as is 
usually supposed to establish a species, between P. guadalupe and the 
extreme form of awhitfieldi. 
If, however, one admits that all American forms make up only one 
species, it becomes illogical to separate the European forms from each 
other or the American from them, and, consequently, all the forms of 
Placenticeras are one species. 
I have not been able to find any middle ground between these two 
extremes, but have found that it is possible to diagnose species by describing 
the normal forms, meaning thereby the shells that are most distinct and 
usually most numerous in each species, and adding thereto the mention of 
intermediate forms. 
When this is done, it is seen that in this genus the separable forms or 
species can be distinguished by their differences in development of the 
sutures, of identical ornaments, and of the sectional outlines of the volutions. 
The changes that take place in the aspect of the venter and of all characters 
in the senile or gerontic stage are included under the term development, 
which in the sense here used means all the modifications of the ontogeny. 
