236 



IOWA GEOLOGICAL SURVEY 



Fig. 35. Scaumenacia eurta (Whiteaves). Upper Devonian; Scaumenac Bay, Province 

 of Quebec. Restoration of headshield based upon a nearly complete individual belonging 

 to the Yale Museum. Compare sensory canals with those of Dipterus, as shown in head- 

 shield figured in preceding illustration, x 1-1. 



Subclass TELEOSTOMI. 



The great group of fishes commonly known under the designa- 

 tions of Ganoids and Teleosts and first recognized by Owen as 

 a single subclass, Teleostomi, makes its appearance in the Lower 

 Devonian, but does not really become significant until the Car- 

 boniferous. The Crossopterygian order, which predominates 

 during the Devonian, is somewhat abundantly represented to- 

 ward the close of that system in this country; but except in 

 Canada, all the remains are extremely fragmentary, consisting 

 of detached scales, teeth, plates, and in two or three instances 

 of imperfectly preserved skeletons. The most important Amer- 

 ican genera are Holoptychius, Sauripterus, Onychodus and 

 Eusthenopteron, but of these the last-named alone has been 

 found in a state bordering upon completeness. Nevertheless, it 

 is possible to frame a tolerably accurate conception of the re- 

 maining genera through comparison of their characteristic parts 

 with the admirably preserved skeletons of their foreign repre- 

 sentatives, especially those from the Scottish Old Red Sand- 

 stone. In this way the detached head plates and bones of the 

 shoulder girdle belonging to Onychodus, for instance, acquire 



