THE ARTHRODIRA, TELEOSTOMII, DIPNOI, AND AMPHIBIA. 387 



form a homogeneous stock that stands distinctly apart from all other primitive 

 vertebrates. In practically all of them the ancestral dermal armor has a pro- 

 longed and flourishing existence, and is rarely, or never, entirely suppressed. In 

 the more primitive groups it survives in the form of large superficial plates 

 ornamented with low rounded tubercles, or with sinuous, beaded ridges. On 

 the trunk and tail they are usually smaller, forming irregular, polygonal, or 

 rounded scales which tend to sink deeper into the skin and eventually to dis- 

 appear, leaving even in such primitive forms as Bothriolepis and some coccosteans a 

 practically naked skin behind. But in the adults of all branches of the phylum 

 a considerable number of the ancient, large-sized cranial plates are retained in 

 the head region, forming a characteristic covering for the roof and sides of the 

 head, jaws, gill chamber, and pectoral arches. With this prolonged survival 

 of the primitive dermal armor, there is an early and vigorous development of an 

 endoskeleton, consisting of true bone, which here makes its appearance for the 

 first time in the history of the animal kingdom. 



A bony floor and sides to the endocranium are formed, as well as complete 

 bony vertebrae consisting of centra intimately united with neural and haemal 

 arches and transverse processes. 



A large peribranchial chamber is always present, but the rigid, armored walls 

 of this chamber, so characteristic of the ostracoderms, are greatly shortened in the 

 arthrodires, and in the teleostomes, dipnoi, and amphibia, give place to membran- 

 ous folds that may or may not be strengthened by movable opercular plates. 



Three pairs of oral arches are usually conspicuous in the embryonic stages, 

 and the branchial arches may retain remnants of arachnid appendages, in the form 

 of external gills, hyoidian "balancers," oral arch papillae, tentacles, or adhesive 

 discs. 



The parietal eye is generally well-developed, and so far as known, a large, 

 lung-like air bladder occurs in all the main subdivisions of the phylum. The 

 primitive pectoral and pelvic appendages may be narrow and elongated, with 

 a bony internal skeleton that in the earliest fish-like descendants of the arthrodires 

 shows, for the first time in the evolution of the vertebrates, distinct traces of the 

 radiate terminal digits, and the jointed axis characteristic of all primitive land 

 vertebrates (Eusthanopteron. Fig. 265.) 



The ova are of moderate size, frequently covered with an adhesive, gelatin- 

 ous substance, and are generally fertilized externally. The antiarcha, coccoste- 

 ans, dipnoi, and primitive teleostomes were preeminently shallow water, shore- 

 loving forms, as are their survivors to-day. Their highly developed air bladder, 

 leg-like fins, specialized breeding habits, and the general structure and mode of 

 life characteristic of the higher members of this phyla, afford an easy anatomical 

 and physiological transition to the amphibia, and hence to the higher air-breathing 

 land vertebrates. 



On the other hand, the young of many amphibia, dipnoi and, teleostomes pass 

 through a larval, or tadpole, stage generally characterized by a large head, by the 



