THE QUARTERLY REVIEW OF BIOLOGY 



that these shells were held erect in life 

 and equally illuminated on all sides. By 

 the same token the animals were ben- 

 thonic crawlers and not swimmers. Not 

 only were they crawlers, but I believe 

 that they crawled by means of a soled 

 foot like that of a gastropod and not by 

 means of their tentacles as does the 

 existing octopus. In fact, were it not for 

 their camerated shells and siphuncles one 

 would be disposed to doubt their cephalo- 

 pod character and consider them gas- 

 tropods. 



My restoration of Cyrtoceras -parvulum 

 Barrande will indicate sufficiently my 

 conception of the animal (Plate 4). The 

 extent to which the soled foot has as- 

 sumed a tentacular form in front is, of 

 course, entirely problematic. If these 

 forms were organized as I have assumed, 

 they would represent a survival, as late 

 as the Silurian, through a continuous 

 ancestry, of the primitive nautiloid organ- 

 ization of the early Cambrian or pre- 

 Cambrian, and would be otherwise un- 

 related to their contemporary cyrtoceran 

 associates. 



Among these cyrtocones with color patterns on 

 all sides of the shell are Cyrtoceras fallax Barrande, 

 with straight transverse bands; Cyrtoceras zebra 

 Barrande, with closely spaced fine subparallel wavy 

 bands; Cyrtoceras intricans Barrande, with somewhat 

 more angular and less regular bands, more pointed 

 backward than forward and perhaps prenuncial to 

 chevrons; Cyrtoceras veteranum Barrande, with closely 

 spaced chevrons; and Cyrtoceras decurio Barrande with 

 large angular chevrons. A sixth species is orna- 

 mented with spots all around. 



ADAPTATIONS FOR A MORE OR LESS PASSIVELY 

 FLOATING MODE OF LIFE 



Adaptations by means of which the 

 animal spent most of its existence in 

 passively floating are various and are 

 attained in unrelated stocks throughout 

 the geological history of the class. 

 Whether they floated near the surface, in 



intermediate depths, or hanging over the 

 bottom — the feeding ground of the exist- 

 ing Nautilus — is immaterial; certainly 

 those forms which retained locomotive 

 powers could readily maintain their po- 

 sition at any desired depth. In the 

 adaptation diagram (Plate z) all floating 

 forms have been placed near the surface, 

 but this is solely a matter of composition 

 on a crowded diagram. The recurved 

 aperture in such an ammonite as Macro- 

 scaphttes, or in some species of Heteroceras or 

 Nostoceras leads to the conclusion that they 

 drifted over the bottom while feeding, 

 but in other genera there is no definite 

 evidence on this point. 



There are a considerable number of 

 orthoconic Nautiloidea, prevailingly small 

 and breviconic, found from the Ordovician 

 to the Carboniferous, and principally in 

 the families Rizoceratidae, Oncoceratidae, 

 Poterioceratidae, and Trimeroceratidae, 

 represented by a variety of genera, and 

 exhibiting numerous differences in their 

 structural details, which floated head 

 downward by reason of the gas filled 

 chambers of the apex of the cone. This 

 type is illustrated in the diagram by 

 Mandaloceras (Plate z, fig. 15), a Silurian 

 genus. 



In Mandaloceras this position must have 

 been maintained from infancy onward. 

 At maturity the aperture of the living 

 chamber was greatly contracted, but the 

 hyponomic funnel probably retained its 

 propulsive powers, since the space in front 

 of it was not roofed over by the inflected 

 margins of the aperture. The expulsion 

 of water from the mantle cavity through 

 the funnel would cause the animal to rise 

 vertically. Many other of these forms 

 had variously contracted apertures leaving 

 only restricted openings for the eyes, 

 tentacles, mouth, and funnel. Many 

 students have thought that this was to be 

 interpreted as indicating that their food 



