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involute Ammonoid, i. c, 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 Turrilites Boblayei^Arietidce) 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 wh#n 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. Zoology, 



