318 
CROSSOPIERYGII. 
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. 
JSynojJsis of Families. 
I. Pectoral fins acutely lobate. 
Vertical infoldings of the walls of the 
teeth very numerous and complex 
(‘ dendrodont ’) ; scales cycloidal . . " HoLOPTYCHiiDiE (p. 321). 
II. Pectoral fins obtusely lobate. 
Vertical infoldings of the walls of the 
teeth comparatively few and simple ; 
scales cycloidal . Khizodontid^: (p. 341;. 
Walls of teeth only slightly infolded at 
the base ; scales rhomboidal. Osteolepid.® (p. 367). 
III. Incertse 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 Khipidistia 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 preciso 
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, subtriangular expansions, which unite mesially and 
embrace the notochord in a groove, which is roofed behind but open 
anteriorly h 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. 
