B. SKELETON. 



I. DERMAL SKELETON. 



THE dermal skeleton, as phylogenetically the older, is best 

 considered before the endoskeleton. Its relative age is shown 



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

 Siluroid) ; b, DENTICLES FROM 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. 



