196 



ANATOMY OF VERTEBRATES. 



underlies the plate d 3, in Acipenser Sturio, so also do the ossified 

 ex- and super-occipitals underlie in PoJypterus the three dermal 

 plates corresponding in position with d 3 in Ac. Sturio. The true par- 

 occipital is equally distinct from the plate marked d 8, in Ac. Sturio 

 and its representative subdivisions in Pohjpterus. The dermal 

 plates in advance of these coalesce with the true parietals, frontals, 

 postfrontals, and part of the mastoids. But the varieties in the 

 dermal plates within the limits of a genus, as exemplified by the 



126 



Furc rmtf of ciido- .'iiid exo-skeletiiii of Stu 



sino-le intorfrontal in Acipenser Sturio, by the three interfrontals 

 in Ac. Scypha, by the divided superoccipital plate in Ac. brcviros- 

 tris, &c., sufficiently warn against the confusion arising from 

 applying to dermal plates the names of the true cranial bones in 

 recent and extinct ganoid and placoganoid fishes. The median 

 cranial ganoid plates in the Sturgeons are plainly a continuation 

 forward of the dermal plates, (ib. d s, fig. 125), of the mid-line of 



the back ; and examples of a like re- 

 petition occur amongst the Osseous 

 Fishes in the dermal epicranial spines, 

 for example, of the Angler (Lophius), 

 which suppi5rt the long fishing-fila- 

 ments upon the liead, or in those 

 modified ones forming the sucking disk 

 on the head of the Remora. 



In certain fishes of the Devonian or 

 Old Red Sandstone ]>eriod the head and part of the trunk were 

 encased by coarticulated ganoid bony jilates. Fig. 127 shows the 

 proportions in which the exo- and endo-skcleton entered into the 

 conservable framework of one of these ancient fishes, termed 

 Coccostei/s (Itdhhos berry, osteon bone), in reference to the tuber- 

 cular enamelling of tlic exterior of the combined helmet and 



