30 ACCELERATION OP DEVELOPMENT IN FOSSIL CEPHALOPODA 



i Showing similarity of 

 young stages, due to 

 adaptation in stocks 

 that are wholly distinct. 

 The young stages are 

 probably coenogenetic. 

 Baculites, PI. XIII, figs. 1-9, a reversionary form from Lytoceras. 



Trachyceras, PL XV, figs. 13-16, one of the most highly specialized of the 

 Ceratitidae; showing the beginning of arrest of development in the 

 prolongation of the entogeny, and persistence of the ancestral Tiro- 

 lit es stage throughout adolescence. 



Clionites, PI. XV, figs. 1-8, a retarded descendant of Trachyceras, rever- 

 sionary by arrest of development toward Tirolites. 



Clionites (Calif ornites), PI. XV, figs. 9-12, a form still more strongly 

 reversionary than the preceding species, with almost complete palin- 

 genesis of Tirolites characters, but with inheritance of the trachy- 

 ceran furrow and sculpture from its immediate ancestor; these are 

 characters that Tirolites never had. 



Otoceras, PI. X, figs. 6 and 7, a transitional Permian genus. 



Paratissotia, PL X, figs. 8-10, a Cretaceous genus, arrested in develop- 

 ment, and showing atavistic reversion to characters very like those of 

 Otoceras. 



Waagenoceras, PL X, fig. 12, a Permian genus, primitive and progressive. 



Sphenodiscus, PL X, fig. 11, a Cretaceous genus, arrested in development, 

 and showing a close approach to the septation of Waagenoceras. 

 These two genera do not belong to the same line of descent, hence the 

 convergence is not due to atavism. 



Heterotissotia, PL X, figs. 2-4, a Cretaceous genus, showing arrest of de- 

 velopment, and reversion to some form like Ceratites, but probably 

 not to any member of the Ceratitidae. 



Ceratites, PL X, fig. 15, a Triassic genus, like the reversionary forms of 

 later Cretaceous groups, the ' ' Pseudoceratites. " 



Neolobites, PL X, fig. 1, a Cretaceous genus, showing arrest of develop- 

 ment and reversion to the Goniatite stage, though probably not to 

 any known Paleozoic genus. 



