19G 



ANATOMY OF VERTEBRATES. 



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

 ex- and super-occipitals underlie in Potypterus 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 Polypterus. The dermal 

 plates in advance of these coalesce with the true parietals, frontal s, 

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

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



-^J^-i < =cx - o-5cx-=3jx3^r^r^' 



Fore part of eiido- and exo-skeleton of Sturgeon 



126 



ft 



single interfrontal in Acipenser Sturio, by the three interfrontals 

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

 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 support the long fishing-fila- 

 ments upon the head, 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 period the head and part of the trunk were 

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

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

 conservable framework of one of these ancient fishes, termed 

 Coccosteus (kokkos berry, osteon bone), in reference to the tuber- 

 cular enamelling of the exterior of the combined helmet and 



Scales of Aniblyptcrus striatus 



