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involute Ammonoid, 7. ¢., it might have been excentric or helicoid. 
So far as known, however, all European series approximate to normal 
forms in the young. Here and there also there are diseased indi- 
viduals, as in the so-called Zurrilites Boblayet (Arietide) and Turril- 
ites. Valdani and Coynarti figured by D’Orbigny, and other isolated 
examples of unsymmetrical shells having helicoid tendencies in the 
ephebic and even younger stages. It is also fully demonstrated by 
specimens and drawings that many Turrilites and _ helicoceran 
forms do not have a retroversal living chamber in the gerontic 
stages as in the scaphitoid and ancyloceran-like series traceable to 
various genera. ‘This may be due to the incompleteness of the 
specimens heretofore collected and the perishability of the excentric 
gerontic volution when present. ‘This hardly accounts for those 
species having contracted living chambers and apertures, the presence 
of which are almost conclusive in favor of the opinion that they 
could not have had retroversal gerontic living chambers. 
These facts and the tendency of the terminal gerontic volution to 
return to the mode of revolution in the same plane and to resume 
the lost bilateral symmetry of the whorl in Emperoceras and Nos- 
toceras show plainly that the helicoid spiral is acquired, adaptive 
pathologic tendency that may come in anywhere as an intermediate 
stage in the ontogeny or phylogeny of any degenerative species or 
series, and is not strictly speaking a normal phylogerontic charac- 
teristic. 
Another interesting result of the discovery of Helicancylus by 
Gabb is that Hamulina, Hamites, and probably also the allied 
Ptychoceras can be definitely characterized as phylogerontic forms 
of phylogerontic series. The author has previously claimed, with 
Quenstedt, that this was the only way to account for the Hamites, 
Ptychoceran and Baculites-like modifications of European forms. 
American forms with helicoid tendencies, like Helicancylus, having 
gerontic stages which differ from true Ptychoceras only in the close- 
ness with which the gerontic retroversal bend is made, afford posi- 
tive evidence in the same direction. 
It should be noted in this connection that these remarks do not 
necessarily imply that Ptychoceras has not a distinct mode of de- 
velopment, an ontogeny of its own and also its own peculiar 
genetic series as may be seen in the remarks on that genus. 
I have had an opportunity to study the gerontic stage of a species 
of Helicancylus in the Whitney Coll., Mus. of Comp. Zodlogy, 
