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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 Ammonitinse. 



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 Macros caphites Ivanii, 

 shells with retroversal gerontic stages derived directly from normal 

 Ammonitinse. 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 Cosmocei-as {Amm.) 

 bifurcatum of the Jura. Neumayr is constantly alluded to as the 

 person who discovered this important biological fact, whereas the 

 credit is due to 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. 



