366 FISHES. 



SIXTH SUB-ORDER— PYCNODONTOIDEI. 



Body compressed, high and short or oval, covered with rhombic 

 scales arranged in decussating pleurolepidal lines. Notochord 

 persistent. Paired fins without axial skeleton. Teeth on the 

 "palate and hinder part of the lower jaw molar-like. Branchi- 

 ostegals, tut no gular plates. 



Extinct. The regular lozenge-shaped pattern of the in- 

 teguments of these fishes is described by Sir P. Egerton thus : 

 "Each scale bears upon its inner anterior margin a thick 

 solid bony rib, extending upwards beyond the margin of the 

 scale, and sliced off obliquely, above and below, on opposite 

 sides, for forming splices with the corresponding processes of 

 the adjoining scales. These splices are so closely adjusted 

 that, without a magnifying power or an accidental dislocation, 

 they are not perceptible. "When in situ, and seen internally, 

 these continuous lines decussate with the true vertebral 

 apophyses." In some genera the " pleurolepidal" lines are 

 confined to the anterior part of the side. 



First Family — PLEUEOLEPiDiE. 



Homocercal. Body less high. Fins with fulcra. 

 Pleurolepis and Homoeolepis from the Lias. 



Second Family — Pycnodontid^. 



Homocercal. The neioral arches and ribs are ossified; fJie 

 roots of the ribs are but little expanded in the older genera, but 

 enlarged in the tertiary forms, so as to simulate vertebrm. 

 Paired fins not lobate. ' Obtuse teeth on the palate and the sides 

 of the mandible; maxilla toothless; incisor -like teeth in the 

 intermaxillary and front of the mandible. Fulcra absent in 

 all the fins. 



These fishes abound in Mesozoic and Tertiary formations. 



