10 ACCELERATION OF DEVELOPMENT IN FOSSIL CEPHALOPODA 



was the safest for the larva. This would make the larval stages of these 

 later forms almost wholly adaptive and ccenogenetie. This is illustrated 

 by the development of ScMoenhacMa, (PI. XIII, figs. 16-21), Placenti- 

 ceras, (PI. XIII, figs. 22-28), and Lytoceras, (PI. XIII, figs. 10-15), all of 

 the Upper Cretaceous, in which the larval stages are very much alike, 

 although the phylogeny of the three genera is very different. Lytoceras 

 goes back in an unbroken genetic series to the Lower Triassic, and prob- 

 ably sprang from some Carboniferous member of the Prolecanitidffi. 

 Placenticeras is a phylogerontic form of the Stephanoceratidffi, most 

 likely an offshoot of the Tropitoidea of the Triassic, and hence of the 

 Glyphioceratidae. If this is true it has every right to resemble the Car- 

 boniferous genus Goniatites. The case of Schloenbachia is not so clear, 

 but it is probable that this genus is an offshoot from the Triassic Cerati- 

 toidea, and hence from still a third Paleozoic phylum, the Gephyro- 

 ceratidffi. 



Sharply contrasted with this uncertain and garbled recapitulation of 

 their ancient history is their positive testimony as to their immediate 

 ancestry. And what is true of these three genera chosen for illustration 

 is true of all Cretaceous Ammonites. This is reflected in the lack of 

 agreement in their classification by various authors, and the utter failure 

 to construct a satisfactory family tree for them. Lytoceras and Phyllo- 

 ceras are the only Cretaceous genera of which we know positively the 

 genealogy ; in fact they are almost the only Jurassic genera of which this 

 is true. 



Unequal Acceleration. 



Useful characters tend to be inherited by the succeeding generations 

 at constantly earlier stages, and finally may appear, in the ontogeny of 

 later groups, simultaneously with characters that belonged to other 

 genera in the genetic series. In other words, the growing young shell is 

 not strictly in sequence Anarcestes, Goniatites, Gastrioceras, Columbites, 

 Tropites, the family line, stretching from Devonian to Upper Triassic, but 

 has in the successive stages some resemblance to each of them, with few 

 characters lost, rather obscured by association with other characters that 

 were not synchronous with them. The characters of later genera do, in- 

 deed, appear successively in ontogeny, but some appear at earlier and still 

 earlier periods of growth, until they may even get back into the larval 

 stages. Thus the keel, which is a late character of the Tropitidte, having 

 been developed only towards the end of the Middle Triassic, is pushed 



