CEPHALOPOD ADAPTATIONS 



103 



shell coiled in an open ascending spiral, 

 its apertural end recurved as in Macro- 

 scaphites, and, I believe, had from infancy 

 the habits described above for the mature 

 Macroscaphites. The genus Heteroceras 

 of d'Orbigny is confined to the Cretaceous 

 period, and all of its species have trochoid 

 spiral shells, but many tend to irregular- 

 ity, and some appear to have been 

 benthonic crawlers rather than pelagic. 

 Such a form as Heteroceras stevensoni appears 

 to me to have had a crawling rather than 

 a swimming or floating habit. 



Such genera as Hamulina d'Orbigny and 

 Ptychoceras d'Orbigny of the older Cretace- 

 ous appear to include highly anomalous 

 forms. During growth their shells are 

 relatively slender chambered orthocones, 

 and if the earlier chambers contained gas, 

 as there are no reasons for doubting, it is 

 impossible to see how they could have 

 been oriented except head downward. 

 At maturity the shell, now much en- 

 larged, bends through an arc of 180 

 degrees and continues its growth in the 

 opposite direction, the animal now facing 

 the apex of the shell. This adult en- 

 largement of the body, faced about as it 

 was, would thus shift the center of 

 gravity backward, so that the animal 

 would now be able to swim in a horizontal 

 position, as I have indicated (Plate z, 

 fig. z6). In some species of Ptychoceras 

 the living chamber extends further back- 

 ward than in the species that I have 

 figured. For example, in the much smaller 

 Neocomian species, Ptychoceras emericianus 

 d'Orbigny, it extends three-fourths of 

 the distance toward the apex of the 

 shell. 



In the genus Ptychoceras the flexed por- 

 tion is in contact with the earlier part of 

 the shell. Hamulina differs merely in 

 that the two parts do not come into juxta- 

 position. Hyatt and other authors speak 

 of such forms as these partially uncoiled 



or secondarily straightened ammonites as 

 "degenerate," but they are obviously not 

 degenerate in any way unless one adopts a 

 creed, and considers the closely coiled form 

 of shell the ideal of perfection. Doubtless 

 the last was an ideal for a swimming 

 habit, and that is why such forms pre- 

 dominate in the geological record of the 

 Mesozoic era, but from the point of view 

 of adaptation a Macroscaphites was more 

 specialized, and in its adult form would 

 have avoided a certain amount of compe- 

 tition which it suffered in its youth. If 

 cephalopods, or other organisms, can have 

 imputed to them any purposes other than 

 to live and reproduce their kind, then, and 

 then only, is it permissible to speak of 

 modifications such as these as degenerate. 



ADAPTATIONS FOR A CRAWLING BENTHONIC 

 LIFE 



In considering the mode of life of the 

 swarming ammonites of the Mesozoic and 

 in endeavoring to find some sort of seem- 

 ingly rational explanation for their re- 

 markably complicated septa, the older 

 naturalists, noting that the growth lines 

 across the venter and the margin of the 

 aperture, when this was preserved, usually 

 showed no trace of a hyponomic sinus 

 such as is present in the majority of the 

 nautiloids, reasoned that there was no 

 hyponomic funnel, and hence that the 

 ammonites as a group were not swimmers, 

 but crawlers on the bottom by means of 

 their tentacles; and that this habit had 

 induced the development of the lobes and 

 saddles in the septa, since this would 

 enable the animal to hold and balance its 

 shell more effectively while crawling. 



This view has been widely accepted, 

 although it is open to insuperable objec- 

 tions. It entirely ignores the meaning of 

 their prevailingly high and often greatly 

 compressed, bilaterally symmetrical, 

 planospiral shells; it ignores the fact that 





