THE ORDOVICIAN PERIOD. 



353 



cholites (Fig. 160, h), which resembled the Nautilus of to-day. The 

 straight forms preponderated, however, and the sutures scarcely de- 

 parted from severe simplicity. We shall find that in later periods the 

 septa and other features varied still more widely, and by so doing 

 marked, in a very tangible way, the progress of the class. 



Perhaps no other invertebrates so well show the progress of Paleo- 

 zoic and Mesozoic times as the cephalopods, and this invites a special 



Fig. 161. — Ordovi&an Gastropods. a, Subulites regularis U. and S.; b, Maclurea 

 logani Salter; c, Lophospira helicteres (Salter); d, Cyclonema bilix (Conrad); 

 e, Schizolopha textilis Ulrich; /, Helicotoma planulata Salter; g, Hormotoma 

 gracilis (Hall); h, Eccyliomphalus triangidus Whitfield; i] , Archinacella cingulata 

 Ulrich; k-l, Ophileta complanata Vanuxern; m, Cyrtjlites ornatus Conrad; n, 

 Raphistomina lapicida (Salter); o, Protowarthia cancellala (Hall); p, Conradella 

 fimbriata U. and S. ; q, Bellerophon clausus Ulrich; r, Conularia trentonensis 

 Hall. The exact zoological position of Conularia (r) is not certainly known, but 

 it is usually regarded as a pteropod. The remaining figures illustrate in a measure 

 the great variety of forms assumed by the Ordovician gastropods, a remarkable 

 amount of differentiation when the slight representation of the group in Cambrian 

 faunas is recalled. 



study of these early forms. The size attained in this early period was 

 probably as great as ever reached by the chambered cephalopods, 

 some of the shells being twelve or fifteen feet in length and a foot in 

 greatest diameter. From this maximum they ranged down to forms 

 smaller than a pipe-stem. Their habits are wholly matters of inference 

 from their structure, and from the habits of their relatives in the present 

 seas. Perhaps they floated, shell uppermost, or crawled upon the 

 bottom, and preyed upon a variety of weaker forms of life. 



