DEVONIC FISHES OF THE NEW YORK FORMATIONS I 59 



actual remains which fulfil the requirements of a common ancestor to the 

 three recognized orders of lung fishes, — Arthrodires, Ctenodipterines and 

 Sirenoids. It may be that confirmation of this nature will never be forth- 

 coming, owing to the imperfection of the paleontological record. Never- 

 theless, guided by the principles of structural resemblances, and not by any 

 exercise of the art of divination, one may project the divergent lines of 

 descent backward until they meet in a common point; and at this point is 

 to be sought the ancestry of the three orders with which we are now 



acquainted. 



Family dif-xk^RIDAE 



Cranial roof bones numerous ; no secondary upper jaw, and no mar- 

 ginal series of teeth above or below ; jugular plates present. Upper and 

 lower dental plates triangular, with outwardly radiating ridges, usually 

 tuberculated or strongly crenulated, rarely smooth or nearly so ; no vom- 

 erine teeth. Caudal fin heterocercal ; two remote dorsal fins opposed to 

 the pelvic and anal fins, separated from the caudal. 



Genus dipterus Sedgwick & Murchison 

 Our knowledge of the complete form of this genus, shown in text figure 

 34, is dependent entirely upon the well preserved skeletons occurring in the 

 Scottish Lower Old Red sandstone. In this country Dipterine remains are 



confined exclusively to detached hard parts, such as dental plates, scales, 

 and calcified labial cartilage. Under these circumstances it is sometimes 

 difficult to determine whether teeth of a certain form should be retained in 



