34:2 PALAEOZOIC SYSTEM OF ROCKS. 



as the Ceratodus, Fig. 438, and the structure resembling a leg is shown 



in Figs. 447 and 448. 



6. The teeth of many Devonian Ganoids are fluted or channeled on 



the outer surface near the base (Fig. 449, a). On section it is found 



that the inner surface next the pulp 

 is deeply folded (Fig. 449, b). This is 

 called labyrinthine structure. It is 

 still more marked in early A mphibians, 

 and may be regarded as an amphibian 



Fig. 449.— Structure of a Ganoid Tooth character. 



(after Agassiz). a. External form, nat- n t\ • tvi • n -n ri 



urai size ; b, enlarged section, showing 7. Devonian Placoids were all Ces- 

 stmcture. tracionts, i. e., they all had cobblestone- 



pavement teeth, instead of the lancet-shaped teeth characteristic of 

 modern sharks. 



Devonian Fishes were Generalized Types. — Teleosts are typical 

 fishes; Ganoids and Placoids, especially Devonian Ganoids and Pla- 

 coids, were both connecting and embryonic types — i. e., along with their 

 distinctive fish-characters they combined others which connect them 

 with higher vertebrates, especially amphibians, and still others which 

 are found in the embryos of Teleosts. The most important connecting 

 characters of Ganoids, especially Devonian Ganoids, are : 1. An ex- 

 ternal protective armor of thick, bony plates or scales, such as were 

 possessed by early amphibians, and by many reptiles of the present time. 

 2. Large conical teeth channeled at the base, and of labyrinthine 

 structure on section. This structure was very marked in early am- 

 phibians. 3. A cellular air-bladder, freely supplied with blood, open- 

 ing into the throat, and capable of being used to some extent as a lung. 

 We do not know that this was true of Devonian Ganoids, but it prob- 

 ably was, since it is true of all their nearest living allies, viz., Lepi- 

 dosteus, Polypterus, Amia, and especially Ceratodus and Lepidosiren. 

 4. In many cases paired fins which had something like jointed legs 

 running through them. 5. The tail-fin vertebrated as in reptiles. 



Combined with these connecting characters are others which are 

 distinctly embryonic — i. e., are found now in the embryos of Teleosts. 

 The most conspicuous of these are : 1. Cartilaginous condition of the 

 skeleton, and even the retention of the fibrous notochord, which pre- 

 cedes in the embryo the segmentation of the vertebral column. 2. In 

 the Placoderms, the ventral position of the mouth, as in the embryo 

 of Teleosts. 3. The vertebrated tail-fin may be regarded as a connect- 

 ing character, since it is possessed by nearly all amphibians and reptiles. 

 But it may be regarded also as an embryonic character, since the tail 

 of a Teleost passes successively through the stages represented by Figs. 

 444_446, being first isocercal, then heterocercal, and finally homocercal. 

 It is doubtless both connecting and embryonic. 



