CEPHALOPOD ADAPTATIONS 



105 



shells, unless they can be referred to habits 

 of life such as I inferred for Heteroceras 

 reussianum (supra), may be considered to 

 have led a benthonic existence, and their 

 shells, although much less diversified, 

 may legitimately be compared with those 

 exceedingly numerous gastropods whose 

 shell plan is an ascending spiral. Instan- 

 ces of a greater degree of reversed adapta- 

 tion are furnished by the marine 

 limpet-like pulmonates Siphonaria and 

 Gadinia, whose ancestors became terres- 

 trial, their descendants of these two 

 genera reentering the sea and becoming 

 benthonic. 



That such modifications of the cephalo- 

 pod shell are not confined to the closing 

 days of ammonite history, but occur also in 

 the Paleozoic nautiloids, is proof that they 

 are adaptations, and not degenerations or 

 phylogerontic features. As I have stated, 

 trochoid shells are far more numerous 

 among ammonoids than among nautiloids; 

 and among the latter are unknown from 

 horizons earlier than the Devonian. This 

 is just what might be expected if cephalo- 

 pod evolution was anything like I have 

 outlined. Not only did they take their 

 origin from a benthonic stock, but it 

 required long ages to acquire the coiled 

 shell. Even as late as the Devonian 2.5 

 per cent of the cephalopods were ortho- 

 cones and, as has been shown, some of 

 these were benthonic, as were also the 

 breviconic cyrtocones, so that there was 

 no especially untilled field on the sea 

 bottom to tempt the adoption of this mode 

 of life among the nautiloids. 



We may also infer from the transverse 

 profiles of the shells, although this is 

 highly problematic, that the nautiloid 

 swimmers as a class were less effective 

 swimmers than the later ammonoids, and 

 fed for the most part near the bottom, as 

 does the existing Nautilus. We may also 

 infer that competition for food was 



keener among the Mesozoic ammonoids 

 than it was among the Paleozoic nauti- 

 loids, or that the food of the latter was 

 more exclusively nectonic, so that there 

 would have been a greater stimulus for 

 the adoption of a benthonic habit in the 

 Mesozoic than in the Paleozoic, although 

 this is very hypothetical. Nor can the 

 influence of the great abundance of mostly 

 benthonic arthropods in the older Paleo- 

 zoic (trilobites and meristomes), or of the 

 arthrodiran, ostracoderm, and true fishes, 

 be ignored in seeking an evaluation; but 

 the influence of these factors, important 

 as they are, is so obscure that I will not 

 attempt to discuss them. 



A secondarily benthonic nautiloid, de- 

 scended from a coiled swimming ancestor 

 is the Devonian genus Trochoceras Barrande. 

 Among the ammonoids the small Coch- 

 loceratinae, as exemplified by such genera 

 as Cochloceras Hauer, or Paracochloceras 

 Mojs., of the marine Triassic, may be 

 mentioned as among the earliest. In 

 the later Mesozoic there are a number of 

 such genera, for example: Turrilites Lam- 

 arck, Emperoceras Hyatt, Helkoceras 

 d'Orbigny, and probably some species of 

 Heteroceras d'Orbigny — all from rocks of 

 Cretaceous age. The first especially often 

 reaches a large size. Two of these 

 secondarily benthonic ammonoids are 

 shown in the adaptation diagram (Plate 

 z, figs. 19 and zo). Doubtless the empty 

 early chambers in these forms so reduced 

 their specific gravity that the energy of 

 handling them was reduced to a minimum. 



The extreme of benthonic adaptation 

 is that of the apparently sessile, or at least 

 static, genus Nipponites, described by Yabe 

 recently from the Upper Cretaceous of 

 Japan, but much fuller information is 

 desirable before attempting to visualize 

 its habits. Nipponites was irregularly 

 coiled like some species of Vermetus or 

 a — a tendency which may be said to 



