CEPHALOPOD ADAPTATIONS 



107 



I do not believe that all of the blame can 

 be fastened upon the hereditary genes. 

 Also, and more clearly, it is to be corre- 

 lated with a more secure connection 

 between the animal and its shell. This 

 is to be rather conclusively inferred from 

 the complementary relationship between 

 the degree of development of adjacent 

 lobes and saddles, where highly modified 

 laterals tend to bring about underdevel- 

 opment in their fellows. 



We are now fairly familiar with the 

 formation of the septa in the existing 

 Nautilus, which have their inception in 

 the epidermis of the hind part of the 

 visceral sac; this first becomes a membrane, 

 is then conchyolinized, and finally calci- 

 fied from the periphery inward on its 

 anterior side next the animal. Hence the 

 horny nidus is left behind the calcified 

 septum. That septa are formed period- 

 ically, and alternately with periods of 

 active metabolism, is inferred from their 

 general regularity of spacing and from the 

 correlation between growth lines of the 

 shell and the spacing of the septa, as is 

 shown, for instance, in Ruedemann's 

 description of the shell of an Ordovician 

 species of Geisonoceras . 



It was long supposed that the flaccidity 

 following the expulsion of the sexual 

 products marked the time of septal for- 

 mation, but as Willey has shown in 

 Nautilus — and I think that his observation 

 may be used for generalization — the last 

 septum is formed before puberty. Perhaps 



septal formation may be correlated with 

 seasonal climatic change, either as a direct 

 factor or through the influence of climatic 

 change upon food supply, although it must 

 be admitted that seasonal climatic varia- 

 tions are minimized in the seas. 



If snakes had shells, the annually shed 

 skins would be accumulated in such a 

 shell exactly (homologously) as are the 

 endocones of the early Endoceratidae, and 

 we may visualize in this periodic shedding 

 of the hardened skin of the apical cone in 

 the earliest nautiloids the beginnings of 

 septal formation. If the extinct cephalo- 

 pods had some muscular connection be- 

 tween the hinder part of the body and the 

 shell in the region where the septum joins 

 the shell wall, such a connection would 

 supplement the comparatively feeble 

 attachment of the lateral and annular 

 muscles, and would influence the crump- 

 ling of the apex of the visceral cone; or, 

 if there was such a connection directly 

 between the body and the septum, any 

 increase in the area of the latter by folding 

 would increase the effectiveness of such a 

 union; or, not to go beyond the well 

 ascertained facts of observation, such a 

 folding of the septum around its periphery 

 — whether of a goniatitic or ammonitic 

 character — into which the periphery of the 

 hinder mantle penetrated, would enor- 

 mously strengthen the bond between the 

 animal and its shell, even though they 

 merely remained in juxtaposition and 

 were not directly united. 



LIST OF LITERATURE 



Abel, O. Palaobiologie. 1911. 



Barrantje, J. Systeme Silurien de la Boheme. 



Vol. z. Cephalopodes. Prague, 1867-1877. 

 Beneckb, E. W. Lebensweise der Ammoniten. 



Abh. Spezialkarte Elsass-Lothringen, N. F., 



Heft 6, 1905. 

 Branco, W. Beitrage zur Entwickelungsgeschichte 



der fossilen Cephalopoden. Palaeont., Bd. 



16-2.7, 1880-1881. 



Bubnopf, S. von. Uber die Lebensweise und das 

 Aussterben der Ammoniten. Naturwissen- 

 schaften, Heft 32., 192.2.. 



Buckland, W. Notiz iiber die hydraulische Wir- 

 kung des Siphos bei den Nautilen, Ammoniten 

 u. a. Polythalamien. Neuesjahrb., 1835. 



Buckman, S. S. Type Ammonites. Vols. 1-5, 

 1909-1916. 





