566 
composite mode of development described below in Nostoceras and 
Emperoceras with a gerontic stage which is a close approximation to 
Hamulina and Ptychoceras,* and affords evidence that this genus is 
a phylogerontic form in which the gerontic retroversal last volution 
replaces the helicoceran. These and other forms appear at any 
rate to give an approximate solution of the difficult problem of the 
derivation of such form as Hamites, Hamulina, Ptychoceras and 
Baculites, and also Turrillites and Helicoceras. 
The helicoid spiral appears sandwiched between a phylogerontic 
nepionic stage in Nostoceras and Emperoceras and a true onto- 
genetic, gerontic living chamber with a retroversal curvature. This 
ontogeny shows this spiral to be a special, probably pathologic adap- 
tive mode of development peculiar to the ephebic stage of some of 
the phylogerontic series, but not necessarily having any correspond- 
ing feature in the gerontic stages of any large number of normal 
formed Ammonitine. 
This explanation is in accord with the fact that all normal Am- 
monoids and Nautiloids revolve in the same plane even in the 
gerontic stage, and enables one to explain the most puzzling of the 
degenerative forms. Thus there may be, as in AZacroscaphites [vanit, 
shells with retroversal gerontic stages derived directly from normal 
Ammonitine. Some helicoceran forms are also derived directly 
from similar normal forms, the most wonderful example being 
the series discovered and accurately described many years since by 
Quenstedt, who traced helicoceran and crioceran, and even bacu- 
lites-like shells all back to their proper origin in Cosmoceras (Amm.) 
bifurcatum of the Jura. Neumayr is constantly alluded to as the 
person who discovered this important biological fact, whereas the 
credit is due te Quenstedt, who showed that all such forms in the 
Jura were probably pathologic derivatives of normal forms. I 
have examined a considerable number of the species of Turrillites 
and Helicoceras from European localities, and although the apices 
of some of these were small enough to have shown at least the be- 
ginning of an excentric nepionic or neanic stage, if any had existed 
I did not succeed in finding any indications of the presence of such 
forms in the young. It is, however, very strange that the youngest 
stages are invariably absent even in large series of specimens of the 
same species, and this suggests that the youngest stage was especially 
liable to destruction, and might not have been like a normal formed 
* See remarks on Ptychoceras. 
