MUSEUM OF COMPARATIVE ZOOLOGY. 81 



The dorsal concavity in the young of Nautilus atratus is marked at 

 the very commencement of the whorls by the flattening of the elliptical 

 outline of the second septum.* In Nautilus Pompilius, however, the 

 dorsum is flattened at an earlier period, the concavity affecting the con- 

 formation of the first septum. 



Sandberger f and Richard Owen J both allude to the Goniatitic 

 stage, and also to a subsequent Ceratitic period. Sandberger's recog- 

 nition of these periods of development, though not quoted by Owen, 

 has priority in point of time, and is much fuller and more comprehen- 

 sive. He also shows that Hauer really first called attention to them 

 by his figures of Amm. floridus. § Sandberger, however, failed in 

 seeing their full significance, since he most emphatically denies that 

 these transformations show any affinity between Goniatites and Nau- 

 tilus in the following words: " Obgleich ich die letztere Thatsache '* 

 (the simple Nautilian characters of the septa of young Goniatites) 

 " keineswegs fiir einen weiteren Nachweis vewandschaftlicher Verhalt- 

 nisse von Goniatites und Nautilus geltend zu machen gedenke und 

 angesehen wissen will." Owen describes the transformations of an 

 Ammonite, only in order to show that they were simpler in the young 

 than in the adult, and that the young of the same species at different 

 periods of growth had been by different authors referred respectively 

 to Goniatites, Ceratites, and Ammonites. 



The so-called Ceratitic stage exists only in the septal sutures, and 

 will be referred to further on. 



After the Goniatitic stage is completed among the typical Ammonites 

 the outlines of the whorls assume no general form, but vary according 

 to the group or genus in which the shell is found. 



SEPTA. 



The first septum of the typical Ammonite is situated at the junction 

 of the first whorl and the ovisac. It has deep, entire, simple lateral 

 lobes on either side, and a prominent abdominal cell as in Nautilus. || 



The dorsal side was not so accurately observed. It is probable, 



* Plate IV, Fig. 6. 



t Oberhessische Gesellschaft fiir Natur-und-Heilkunde, 1858. 



J Palaeontology : Second edition, p. 99. 



§ Cephalopod, von Bleiberg. Plate I, Fig. 14. 



|| Plate I, Figs. 2, 5. 



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