﻿318 CROSSOPTERYGII. 



Suborder II. RHIPIDISTIA. 



Notochord more or less persistent. Axonosts of each of the 

 dorsal and anal fins fused into a single piece ; baseosts much fewer 

 than, and overlapped by, the dermal rays in all the median fins. 



Synopsis of Families, 



I. Pectoral fins acutely lobate. 

 Vertical infoldings of the walls of the 



teeth very numerous and complex 



(' dendrodont ') ; scales cycloidal . . Holoptychiidje (p. 321). 



II. Pectoral fins obtusely lobate. 

 Vertical infoldings of the walls of the 



teeth comparatively few and simple ; 



scales cycloidal RmzoDONTiDiE (p. 341 J. 



Walls of teeth only slightly infolded at 



the base ; scales rhomboidal Osteolepidje (p. 367). 



III. Incertae Sedis. 



Tooth -structure simple; a dentigerous 

 presymphysial bone ; scales cy- 

 cloidal Onychodontidje (p. 391). 



The osteology of some members of each of the three typical 

 families of Ehipidistia is now tolerably well known, as the result 

 especially of researches by Pander, Huxley, and Traquair. There 

 is a remarkable uniformity in the arrangement of the bones and 

 fins, and a brief summary of the chief structural features may be 

 presented as follows. 



The cranial cartilage is in some degree ossified, but the precise 

 arrangement and extent of nearly all the tracts remain still 

 unknown. It suffices to remark that in Megalichthys (Ectosteo- 

 rhachis) the parachordal cartilages are ossified in the form of a pair 

 of large, subtri angular expansions, which unite mesially and 

 embrace the notochord in a groove, which is roofed behind but open 

 anteriorly \ The whole of the cranium, however, is covered with 

 thick dermal plates, which exhibit a definite symmetrical disposi- 

 tion except towards the extremity of the rostrum ; and there is, 

 similarly, a considerable development of membrane-bones on the 

 roof of the mouth. The shield of the cranial roof is divided by a 

 much-pronounced, transverse suture into a parietal and frontal 

 moiety, the latter being usually the smaller, and excavated on each 

 side to form the upper border of the orbit. The parietal portion of 

 the shield consists chiefly of a long, narrow pair of parietal bones, 



i E. D. Cope, Proc. Amer. Phil. Soc. vol. xx. (1883), p. 628. 



