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substages have been described, although not defined according to the 

 nomenclature used in these pages, by Wurtenburger 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 Oolite," and by the author in 

 the Genesis of the Arietidce. 



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 Tropites 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 

 PI. iii. 



Fig. 13 shows a complete young shell which is in the neanic stage 

 of growth. Fig. 1 7 is a 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, 

 PI. 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., Paleonlogr., xxvi, PI. v. 



