CEPHALOPOD ADAPTATIONS 



was microscopic, and Abel has dubbed 

 such forms microphagous. I see no reason 

 for limiting the size of their food except 

 by the size of the opening through which 

 it had to pass (Plate 5). 



The existing Nautilus is a voracious 

 carnivore, very catholic in its taste. 

 Doubtless the size of the aperture in such 

 a fossil form as Mandaloceras imposed 

 somewhat narrow limits with respect to 

 size of the food particles, since it could 

 not excert its jaws and tentacles through 

 so narrow an opening and tear large 

 sized prey to pieces. The contracted 

 aperture was not, however, an adaptation 

 due to feeding habits, but exactly the 

 opposite, the contracted aperture having 

 originated as a protective device which 

 also served to keep the animal from 

 becoming dislodged from its living cham- 

 ber, exactly comparable with the apertures 

 in such land snails as Polygyra, Anastomia, 

 and Pythia. Such a form as Mandaloceras, 

 if its aperture had been an open one, would 

 have been a juicy morsel for its larger and 

 more actively swimming contemporaries, 

 and to call such apertures gerontic fea- 

 tures is about as absurd as Schmidt's 

 suggestion that the young may have been 

 protected within the living chamber of 

 the mother by them. 



Many of these early floaters had shells 

 which were cyrtocones, often endogastric, 

 as in the Silurian nautiloid Phragmoceras, 

 shown in the diagram (Plate z, fig. 14). 

 These were in general somewhat larger 

 forms than the breviconic orthocones like 

 Mandaloceras, but like them they retained 

 the functional hyponomic funnel, and had 

 greatly constricted apertures during their 

 adult life. What was said of Mandaloceras 

 applies also to forms like Phragmoceras, 

 except that the latter could probably 

 propel themselves horizontally. To rise 

 or sink they would have to depend on the 

 extension or contraction of the arms to 



induce changes in their specific gravity, 

 unless, like the existing Spirula, they 

 could point their funnel in different 

 directions. 



Another series of evolutionary changes 

 in the direction of adaptations for floating 

 is exhibited by those nautiloids whose 

 ancestors had completely coiled shells 

 (ophiocones). These may have open 

 apertures, as in Schroederoceras (Plate 2, 

 fig. 12.), an Ordovician genus; or con- 

 tracted apertures, as in Ophioceras (Plate 5, 

 fig. z), a Silurian genus. They are con- 

 trasted with such adaptations as Manda- 

 loceras or Phragmoceras in that they were 

 normal swimmers with normally coiled 

 shells during adolescence. At maturity 

 the living chamber became partially free 

 (uncoiled), the margins being built out- 

 ward to accommodate the growth of the 

 animal, which was then strikingly elon- 

 gated as compared with the short bodied 

 Mandaloceras or Phragmoceras , and must 

 have passed the remainder of its life 

 suspended obliquely in the water — the 

 weight of the body causing the shell to 

 tilt forward. 



The end product of such an evolutionary 

 adaptation as Ophioceras, in which it is 

 prenuncial, is exhibited by the nautiloid 

 genus Lituites Breyn., which in early 

 Paleozoic time (Ordovician) set an ex- 

 ample of modification that was repeated 

 in substantially the same way by ammo- 

 noid genera like Macroscaphites, eons later. 

 Lituites was a fairly large form in which 

 the first three whorls form an ophiocone, 

 indicative of its ophioconic ancestry, as 

 well as its normal swimming attitude 

 during adolescence. After this stage of 

 its ontogeny the diameter of the shell 

 increases rapidly and it is nearly straight 

 in its growth for a distance about 14 times 

 the diameter of the coiled early portion. 

 The aperture has a shallow hyponomic 

 sinus, narrow ventro-lateral crests, broad 



