CROSSOPTERYGII. 



77 



almost identical, each having three baseosts supported by the broad end 

 of a single axonost, which is directly apposed to a single arch of the axial 

 skeleton. There are calcified ring-vertebra) in the abdominal region. 



FIG. 58. 



Eusthenopteron foordi '; restoration by J. F. Whiteaves, one-quarter nat. size, 

 scales omitted in caudal region to expose axial skeleton and bases of median 

 fins. U. Devonian ; Scaumenac Bay, Canada, cl., clavicle ; fr., frontal ; 

 i.cl., infraclavicle ; mn., mandible; mx., maxilla; op., operculum ; pa., 

 parietal; g.cl., supraclaviole ; s.op., suboperculum ; s.t., supratemporal ; 

 sb.o., suborbital ; or, cheek-plate. 



Through Rhizodopsis and Gyroptychius the Rhizodontidae 

 pass almost imperceptibly into the Osteolepidae, which have 

 rhombic scales and usually exhibit the fusion of the bones in 

 the frontal portion of the skull and in the mandible. The 

 external bones and scales are covered with a more or less 

 continuous layer of ganoine, often punctate; and in the four 



Fio. 59. 



Osteolepia microltpidotu* ; outline of cranial roof, somewhat enlarged. L. Old 

 Bed Sandstone; Caithness. /., frontal; orb., orbit; pa., parietal; pt.f,, 

 postfrontal ; pt.o., postorbital ; r., fused plates of rostrum ; s.t., supra- 

 temporals; sq., squamosal. Dotted lines mark mucus-canals. 



