CEPHALOPOD ADAPTATIONS 



93 



existing Spirula, which is itself related 

 to the ancestral squids, is quite unlike the 

 remote shelled ancient stock; so that the 

 •existing Nautilus alone can serve as a 

 point of departure from which to envisage 

 the structure and habits of the majority 

 of fossil forms. 



This lack of existing comparable forms 

 has led to serious misinterpretations of 

 extinct forms, stratigraphic and taxo- 

 nomic paleontologists usually not having 

 seen the forest because of the trees, and 

 has opened the way for a multitude of 

 diverse speculations and generalizations, 

 more often dogmatic than probable. It 

 is proposed in the following pages to pass 

 in review what may be logically inferred 

 regarding the structure and habits of 

 these long vanished races, and it is freely 

 admitted that a single fact of observation 

 may upset a sheaf of deductive philosophy. 



CEPHALOPOD CLASSIFICATION 



Richard Owen, who gave us an admir- 

 able account of the anatomy of the pearly 

 Nautilus in 183Z, divided the existing 

 cephalopods into two sub-classes — the 

 two-gilled, shell-less Dibranchiata, and 

 the four-gilled Nautilus or Tetrabranchi- 

 ata. Paleontologists have rather gener- 

 ally assumed that the extinct nautiloids 

 and ammonoids had four gills as in the 

 single surviving genus Nautilus, although 

 the two-gilled forms were obviously de- 

 rived from the same stock. This assump- 

 tion is highly illogical and equally 

 improbable. It is not a matter of great 

 moment whether the extinct forms had 

 two, four, or some other number of gills — 

 there is a great amount of diversity in this 

 feature throughout the molluscan phylum; 

 but it is important, it seems to me, not to 

 base their segregation into major groups 

 upon the number of gills when we can 

 never hope to know what the number was 

 in 99 per cent of the cases . The sub-classes 



of Owen are therefore meaningless 

 throughout all time but the present. The 

 alternative terms Ectocochlia and Endo- 

 cochlia for the two classic sub-classes are 

 not particularly euphonious, fail to 

 recognize the threefold diversity of the 

 cephalopods, and are most inappropriate, 

 since a considerable number of the Endo- 

 cochlia lack all traces of a shell, and 

 others — the extinct belemnoids and the 

 existing Spirula — are ectocochlia in their 

 youth and become endocochlia during 

 their ontogeny. 



Three subclasses should be recognized, 

 namely: the Nautiloidea, Ammonoidea, 

 and Coleoidea or Dibranchiata — the last 

 largely living, and the fossil forms fur- 

 nishing enough indications of their soft 

 anatomy in the fine grained muds of the 

 Jurassic to give a fairly clear idea of their 

 structure. 



A second rather general misconception 

 of another morphological feature of the 

 extinct animal has been the assumption 

 that since the pearly Nautilus has numer- 

 ous tentacles, all fossil nautiloids and 

 ammonoids were similarly equipped. We 

 know that in Spirula, the existing squids 

 and Octopoda, and the extinct belemnoids, 

 quite a different and more restricted 

 number of more specialized tentacles was 

 universally present, and since these groups 

 were deriyed from the more primitive 

 shelled stock, the question of whether 

 fossil nautiloids and ammonoids had many 

 tentacles or a few so-called arms is to be 

 determined, if at all, by evidence drawn 

 from the geological record and not from 

 their supposed position in the taxonomic 

 scheme of the systematists. 



The keynote of evolution of the hosts of 

 extinct cephalopods, as it appears to me, 

 is adaptation — a thought already ably 

 voiced by Diener. The founders of the 

 more modern study of fossil cephalopods — 

 men like Alpheus Hyatt — were entirely 





