EVOLUTION OF FOSSIL CEPHALOPODA. 



253 



descent ; Glyphioceras goes through distinctly the stages 

 from Anarcestes of the Devonian, through Tornoceras 

 and Prionoceras, finally stopping in the Glyphioceras stage. 

 Schloenbachia hastens through the Anarcestes, Tornoceras, 

 and Prionoceras so that they are hardly recognisable, 

 and gets to be a Glyphioceras even in its middle larval 

 stage, goes through two more goniatite stages, and 

 several ammonite stages before it becomes a Schloen- 

 bachia. 



DEVELOPMENT OF GLYPHIOCERAS. 



Glyphioceras in its development does not show the 

 Bactrites (Plate V, Fig. 7) and Mimoceras (Plate V, Fig. 

 8) stages, so these must be studied in lower forms ; but 

 from the Anarcestes stage up they are sharply defined. 

 It begins life as a protoconch or embryonic shell while 

 undoubtedly still in the egg ; this (shown on Plate I, 

 Figs. 1-5) corresponds to the primitive cephalopod. At 

 the beginning of its larval stage the animal left the 

 protoconch, built up the first body chamber, and cut off 

 the embryonic part of the shell by the first septum 

 (Plate I, Figs. 6 and 10, Plate IV, Fig. i, i) ; at this stage 

 the shell is analogous to the primitive nautiloid, and it 

 is called in Hyatt's nomenclature ananepionic. With 

 the second chamber the young shell becomes an ammo- 

 noid, and corresponds to the genus Anarcestes of the 

 Lower Devonian ; this is shown on Plate I, Fig. 6, and 

 on the second and third septum of Fig. 9, also Plate IV, 

 Fig. i, second and third sutures. On Plate IV, Fig. 7 

 shows for comparison the septa of Anarcestes subnautilinus 

 Sandberger. At the fourth suture the shell is transi- 

 tional to Tornoceras of the Middle Devonian. Plate I, 

 Fig. 9, shows the beginning of the Tornoceras stage, 

 which lasts through the sixth chamber, as shown on 

 Fig. 12, and on Plate IV, Fig. i, at the fourth and fifth 

 septa. For comparison the development of the septa of 



