526 



GEOLOGY 



known, contested their supremacy, they were doubtless the undis- 

 puted 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 and so widely differentiated from allied forms 

 as to render it probable that they had already lived a long time. 

 Their general aspect is seen in Fig. 387. The dominant form, as 



Fig. 388. Ordovician Gastropods: a, Subulites regularis U. and S.; 6, Mac- 

 lurea logani Salter; c, Lophospira helicteres (Salter); d, Cyclonema h/li.r 

 (Conrad); e, Schizolopha textilis Ulrich; /, Conularia trentonensis II;ill; 

 g, Hormotoma gracilis (Hall); h, Eccyliomphalus triangulus Whit field: 

 i, Helicotoma planulata Salter; /, Cyrtolites ornatus Conrad; k, Raj>hix- 

 tomina lapicida (Salter); I, Protowarthia cancellata (Hall); m, Bellero- 

 phon dausus Ulrich; n and o, Archinacella cingulata Ulrich. 



well as the most primitive one, was the Orthoceras (Fig. 387, c and 

 /), whose shell consisted of a long, straight, gently tapering cone 

 divided into chambers by plane septa, and connected by a central 

 tube (the siphuncle). Even in the Ordovician period there was a 

 wide departure from the ideal simplicity of this genus. There were 

 curved forms and coiled forms, some of which resemble the Nautilus 

 of to-day (Fig. 387, e). Straight forms predominated, however, 



