248 FOOT-NOTES TO EVOLUTION. 



going through a Tirolites stage in its early youth. 

 Neither does the first discovery of a genus always cor- 

 respond with its first appearance. For example, tere- 

 bratuloid brachiopods are not known below the Devo- 

 nian, but their existence in the Silurian is necessitated by 

 the occurrence of a terebratuloid stage in the ontogeny 

 of spire-bearing brachiopods {Ancylobranchia) of that age. 



One great drawback to this work is that the am- 

 monite faunas of the various ages have been classified 

 by different specialists and on different principles, but 

 all artificial. Thus the Triassic ammonites are divided 

 into Leiostraca (smooth-shelled) and Trachyostraca 

 (rough-shelled), a classification that can not be extended 

 even to Jurassic groups. The Trachyostraca are further 

 divided into TropitidiZ, with long body chamber, and Cerati- 

 tidce, with short chamber. But neither of these groups is 

 monophyletic, for it is quite probable, judging from their 

 ontogeny, that members of both groups are derived 

 from the Goniatitidce, and others from the Prolecanitidce. 

 Further, the authorities agree in deriving the Tropitidce 

 from the Glyphioceratidce, but the larval stages of some 

 of the Tropitidcz show the undivided ventral lobe and 

 an unmistakable resemblance to certain Frolecamtidce ; 

 other so-called TropitidcB show the divided ventral lobe 

 at an early age, and a decided resemblance to the stock 

 of Glyphioceratidce. 



In the same way most authorities agree that the Tra- 

 chyostraca were all extinguished at the end of the 

 Trias, and that all the Jurassic and Cretaceous ammon- 

 ites, with the exception of Lytoceratidce and Phyllocera- 

 tidcE, were derived from the radicle Psiloceras, and this, 

 too, in spite of the fact that many of the genera are 

 rough shelled, and in their larval stages show marked 

 likeness to trachyostracan genera. Any naturalist can 

 convince himself of this by looking at the young stages 



