32 COMPARATIVE ANATOMY. 



the whole skin, and which exhibit exactly the same structure as 

 the teeth proper, which will be described later. 



Such dermal denticles are found in the skin ofElasmobranchs, 

 Ganoids, Siluroids, and Dipnoans; the large shields, which, in 

 the armoured Ganoids and Siluroids (Fig. 19), Lophobranchii and 

 Plectognathi, become united to form a strong bony cuirass, may be 

 derived from the gradual fusion of the above-mentioned basal- 

 plates to form bands and networks. One may even extend this 

 still further, and derive phylogenetically allthe scales of Fishes, 

 as well as the investing bones of the pectoral arch (e.g. 

 Teleostei) and of the primordial skull in the same manner (cp. 

 Fig. 53). 1 



Fossil genera of Amphibia have bequeathed but slight traces 

 of this strong dermal armour to the existing forms of the group : 

 as examples may be mentioned the bony plates in the skin of the 

 back of certain Anura (Ceratophrys dorsata and Ephippifer auran- 

 tiacus), as well as the scales lying between the ring-like scutes of 

 the footless Amphibia (Gymnophiona) (comp. p. 20). The latter 

 may be derived from such a scaly covering as that of the ancient 

 Salamander (Discosaurus) of the Carboniferous formation. 



The dermal skeleton of fossil Reptilian genera, as, for instance, 

 of many Ornithoscelida (Stegosaurus), was still more highly 

 developed. In these, enormous bony plates and spines, sometimes 

 as much as 63 centimetres long, were present in the dorsal region. 

 Teleosaurus also, as well as the Triassic Aetosaurus ferratus, 

 possessed a strong exoskeleton. Amongst existing Reptiles, the 

 Crocodiles, many Lizards (Anguis, Cyclodus, Scincus), and more 

 especially the Chelonia, exhibit a well- developed dermal skeleton. 

 In the latter group a dorsal and ventral shield (carapace and 

 plastron) consisting of numerous pieces and completely enclosing 

 the body must be noticed. Both arise independently of the endo- 

 skeleton, which is preformed in cartilage, that is to say, they are true 

 exoskeletal membrane bones (cp. note on p. 62) ; the exoskeleton, 

 however, comes into the closest relation with the endoskeleton, and 

 may supplant it here and there : thus, in Testudo, for instance, the 

 thoracic and lumbar vertebrae and ribs become quite rudimentary. 



Birds have no dermal skeleton, as already mentioned in the 

 chapter on the integument. 



It is uncertain whether the dermal skeleton present in Arma- 

 dillos (Loricata) 2 only among Mammals is to be derived directly 



1 The dermal denticles of Elasmobranchs are often spoken of as placoid, and 

 the firmly-jointed scales of Lepidosteus and Polypterus as ganoid scales : both are 

 covered by a layer of enamel (probably) developed from the ectoderm, and thus both 

 epidermis and derma take part in their formation. Some Fishes (e.g. Electric Ray, 

 Spatularia, some Eels) are scaleless. 



2 In Armadillos the dermal skeleton consists of a series of transverse bony 

 scutes, which are movable on one another, while in Glyptodon, a fossil member of 

 this group, the dermal plates were firmly united together to form a large shield, 

 which covered the whole body. 



