3G PEOF, II. H. swiyyEKTOX AXD :NrE. A. E. TErEiiAX Tvol. Ixxui 



were complete in all sutrural details and, though only formed 

 in part, were situated at the normal distance from the preceding- 

 septa. These facts run counter to, the views of Eiefstahl ^ 

 supported by Buckman ^ ; and, along with the observations of 

 Willey (1902) upon Xautihis, they furnish further support, if 

 such be needed, for those of Bather ^ and Appellof.^ According to 

 Riefstahl, the last septum was formed in contact with the previous 

 one, and became separated by the growth of the intervening 

 shell- wall. He based his view upon the microscopical study 

 of tlie sepion. This, however, is so highly modified that it would 

 not be safe to use it for throwing light upon the formation of the 

 septum of the (comparatively) closely-allied l)elemnite, still less 

 upon that of the only very distantly-related ammonite. 



YII. The Deyelopmext of the Septum of 

 Dactylioceeas commcxe. 



(1) The Second Sejitum. 



A wax model enlargement was made of the second septum, and 

 from this a contoured plan (fig. 6) and a grajDh (fig. 7), like those 



Fig. 6. — Contoured ])^ an of the second septum o/ Dactylioceras 

 commune shoivn in PI. Ill, Jic/. 4. 



[Explanation as in fig. 4 (p. 32). The contours are drawn at intervals 

 of O'Ol mm.] 



already described for the adult septum, were constructed. The 



^ ' Die Sepienschale & ihre Beziehung zu den Belemniten ' Palfeontographica, 

 vol. xxxii (1886) p. 20. 



- ' Notes on Nautili & Belemnites ' Proc. Geol. Soc. vol. xlvii (1891) p. 165. 



=* ' Shell-Gro^vth in Cephalopoda ' Ann. Mag. Nat. Hist. ser. 6, vol. i (1888) 

 ■p. 298. 



■* ' Die Schalen von Sepia. Spinila, & Navtilus' Kongl. Svenska Yetenskaps- 

 Akad. Handl. vol. xxv, No. 7 (1893) p. 28. 



