386 



THE VERTEBRATES 



tion, the anal claspers of the males, and the absence of an air bladder, indicate 

 an affinity, although probably a remote one, with the elasmobranchs. 



III. The Arthrodira, Teleostomii, Dipnoi, and Amphibia. 



The antiarcha, at some time probably not later than the early Devonian, gave 

 rise to the arthrodira. From the latter sprang the teleostomii and dipnoi, and 



Fig. 259. F"5- 260. Fig. 261. 



Figs. 259, 260, 261. — Figures illustrating three important stages in the evolution of the head and oral arches of 

 vertebrates, as shown by an ostracoderm (Bothriolepis), an arthrodire (Coccosteus) and a, primitive dipnoan 

 (Scaumenacia) . The important steps are: (i) the union of the three pairs of oral arches to form an unpaired upper 

 and lower jaw, with denticulate jaw plates; (2) the separation of the olfactory organs and lateral eyes, and their 

 migration to their typical position in vertebrates; (3) the breaking up, the reduction, and the more intimate union 

 with the head, of the dermal armor to the visceral and respiratory organs, and their transformation into the oper- 

 cular plates that cover only the respiratory organs. 



C and D are semi-diagrammatic restorations, based on the descriptions of Dean, Traquair, Hussakof, and 

 Jaeckel; E and F are restorations of Scaumenacia curta (Whiteaves), made from a large number of well preserved 

 specimens in the author's collection. In F, the mandibular plate, d, is removed on the left, exposing the under 

 surface of the large dental plate of the lower jaw, and the pre-maxillary and maxillary plates of the upper jaw. 



from them, near the beginning of the carboniferous, the first air-breathing verte- 

 brates, or amphibians from which, at some subsequent period, all the higherN 

 vertebrates had their origin, directly or indirectly. (Fig. 309.) 



These animals, the most vigorous and varied offspring of the ostracoderms, 



