352 



GEOLOGY. 



life were the cephalopods, which seem to have come in with extraor- 

 dinary suddenness. Unless the fishes, of which so little is known, 

 contested their supremacy, they were doubtless the undisputed masters 

 of the sea. Their relics first appear at the time of the transition from 

 the Cambrian to the Ordovician, but they were then so far advanced 

 in their development and so widely differentiated from allied forms as to 

 render it probable that they had really lived much earlier. Their 

 general aspect may be gathered from the group in Fig. 160, but only 



Fig. 160. — Ordovician Cephalopods. a, Poterioceras apertum Whiteaves; b, Cyrto- 

 ceras neleus Hall; c, Orthoceras bilineatum Hall; d, Clinoceras mumice forme 

 (Whitfield); e, Orthoceras sociale Hall; /, Gyroceras duplicicostatum Whitfield; 

 g, Oncoceras pandion Hall; h, Trocholites ammonius Conrad. These figures illustrate 

 different types of Ordovician cephalopods, varying from the straight orthocera- 

 tites (e) to the closely coiled nautiloid type (h). In all of them however, the sutures 

 are straight or nearly so. (Weller.) 



small forms, or large forms on a greatly reduced scale, are available 

 for illustration. The dominant form, as well as the most primitive 

 one, was the Orthoceras (Fig. 160, e), whose shell consisted of a long, 

 straight, gently tapering cone, partitioned by plane septa into chambers 

 connected by a central tube (siphuncle). Even in Ordovician times 

 there had already been a wide departure from the ideal simplicity of 

 the Orthoceras. There were not only moderate curvatures, as exem- 

 plified by the Cyrtoceras (Fig. 160, b), but there were loosely coiled 

 forms as the Gyroceras (Fig. 160, /) and close-coiled ones, as the Tro- 



