B. SKELETON. 



I. DERMAL SKELETON. 



THE dermal skeleton, as phylogenetically the older, is best 

 considered before the endoskeleton. Its relative age is shown 



a 



-z 1 



FIG. 17 (after 0. Hertwig). a, DERMAL ARMATURE OF Hypostoma commune (n 

 Siluroid) ; b, DENTICLES FKOM THE SKIN OF THE ABDOMEN OF Callichthys ; c, 

 PLATES FROM THE TAIL-FIN OF Hypostoma. 



Z, dermal denticles, shown broken off from their bases at Z^ ; BP, basal plate. 



not only by Palaeontology, 1 but also by Ontogeny, inasmuch as 

 calcifications and ossifications in the derma or perichondrium 



1 As examples of ancient forms which were protected by well-developed dermal 

 skeletons, may be mentioned the armoured Fishes of the Devonian and Silurian 

 strata, and the armoured Amphibians of the Carboniferous, Trias, and Jurassic. 



