EBPTILES IN GEOLOGY 



1S3 



these, tmless there was a cross membrane also ; and 

 the creature may have thereby actually sailed before 

 the breeze. Those that did not have the cross-arms 



Fig. 73. — Seaperornis regalia, x A (restored by Marsh). 



may havie tacked by means of the single spread of the 

 broad back-fin. This latter would be useful, how- 

 ever, as the dorsal fin of a fish is, in keeping the body 

 steady while the tail lashed (Fig. 75). 



