AIT 
substages have been described, although not defined according to the 
nomenclature used in these pages, by Wiirtenburger in his essay re- 
ferred to above; by S. S. Buckman in his extensive and monumental 
work published by the Paleontographical Society in their volume 
for 1891 on the ‘‘ Ammonites of the Odlite,’’ and by the author in 
the Genesis of the Arietide. 
The gerontic stage has also been fully described and separated 
into two subdivisions by Mr. Buckman and the author, and is easily 
distinguished from the ephebic by the external characters, and as 
stated above the septa become more or less approximated in the 
paragerontic substage and there is often slight but perceptible de- 
generation in the sutures. 
All of the remarks made above apply well enough in a general 
sense to the progressive series of the Ammonoidea, but although 
we know the younger stages of only a few species of the retrogres- 
sive species, there are indications that they will require modifica- 
tions to be true also for the phylogerontic forms. 
Thus Choristoceras (of) Henseli, as figured by Branco,* has appar- 
ently a considerable number of sutures having the undivided ven- 
tral lobe. These are less in number than in some progressive forms 
like Zyropites subullatus, figured on the same plate, but unluckily 
the immediate ancestors of this species are unknown and exact 
comparisons cannot be made. 
The young of the uncoiled forms of the Ammonoidea show 
however, in all their characters that the early inheritance of 
gerontic tendencies interferes with and delays the development of 
the progressive, more complicated structures of the forms from 
which they must have been derived. This is admirably shown in 
the drawings of Dr. Brown, some of which are reproduced on 
Bly iit. : 
Fig. 13 shows a complete young shell which isin the neanic stage 
of rowth. Fig. 17 isa restored side view of the protoconchial stage 
and ananepionic substage with aperture. Fig. 16 gives front view of 
the first volution in the paranepionic substage which begins at the 
fourth septum, and Fig. 18 side view at the sixth septum. Fig. 1, 
Pl. iv, shows the sutures for the same age. 
Figs. 14-16 show the gradual diminution of the area of the con- 
tact furrow and the decrease in lateral diameters ,of the volution 
* Op. cit., Paleontogr., xxvi, Pl. v. 
PROC. AMER. PHILOS. SOC. XXxII. 148. 3A. PRINTED JUNE 25, 1894. 
