18 ACCELERATION OF DEVELOPMENT IN FOSSIL CEPHALOPODA 



parable to Neolobites, and probably none like it ever existed. Its 

 characters are a combination handed down from various members of 



its long family line. 



Other genera of the Cretaceous have ceratitic septa, and here 

 again we have a reversion by arrest of development to an older type of 

 structure. It is not likely that these "Pseudoceratites" are really 

 reversions to the genus Ceratites, for they appear to belong to several 

 different phyla, in which the stage of development with serrated lobes 

 was present in either Permian or Lower Triassic time. The resemblance 

 is so marked that Steinmann* regards Heterotissotia (PI. X, figs. 2-4) of 

 the Cretaceous as a direct descendant of Ceratites of the Middle Triassic, 

 and does not even regard it as a case of atavism, or arrest of development, 

 but simply a persistence of the genus, intermittent because of our lack 

 of knowledge of the intervening forms. No doubt there are numerous 

 gaps in our existing records of extinct faunas, and it is premature for 

 us to be too positive in our denial of the possibility of this being the 

 correct explanation. But that it is extremely improbable nearly all 

 paleontologists will agree. Steinmann compares Heterotissotia with 

 Ceratites semipartitus, which according to Philippi* is a somewhat 

 degenerate type, already reversionary, and probably not an ancestor of 

 later forms. It is, then, more probable, if in the "Pseudoceratites" we 

 have a case of atavism, the reversion is to some still older member of 

 the Ceratitoidea, the Meekoceras group, for instance Asptdites or 

 Koninckites, of the Lower Triassic. 



In any case, whether it is due to atavism, or to independent develop- 

 ment of the same characters in different stocks and in widely separated 

 times, this is a remarkable case of parallelism. Another of the "Pseudo- 

 ceratites," Sphenodiscns, of the Upper Cretaceous (PI. X, fig. 11), 

 approaches closely to the septation of the primitive Arcestoidea of the 

 Permian, especially Waagenoceras (PI. X, fig. 12) and Cyclolohus. Also 

 here there is no probability of atavism, for the phylum of the Arcestidae 

 seems to have died out at the end of the Triassic, and the affinities of 

 Sphenodiscns seem to point to a relationship with the Jurassic Stephano- 

 ceratidae, which certainly did not come from the Arcestidae. 



♦Sitzungsber. Niederrhein. Gesell. fur Natur- und Heilkunde zu Bonn. 

 Naturwiss. Abthell. 1909. Probleme der Ammonlten-Phylogenie (Gattung Hete- 

 rotissotia), pp. 1-16. 



•Die Ceratiten des oberen deutschen Muschelkalkes, Pal. Abliandl. Bd. VIII, 

 1901, Heft 4, p. 357. 



