THE ARTHRODIRA. 



391 



form movable opercula, and those in the ventral wall become attached to the 

 mandibular and hyoid arches to form the gular plates. 



The dermal plates on the dorsal surface of the head (mesocephalon) increase 

 in number, and approach the typical arrangement seen in the primitive air-breath- 

 ing vertebrates. The base of the endocranium becomes ossified; bony centra 

 appear in the sheath of the notochord and, uniting with the neural and haemal 

 arches, form true vertebrae. 



A B 



FIG. 265. Photograph of the left pectoral appendage of Eusthanopteron fordi (Whiteaves). The skeleton of 

 the basal portion of the appendage was exposed; that of the terminal portion was apparently covered by skin 

 which has shrunken sufficiently to show the arrangement of the internal skeleton. The skeleton of this appendage 

 resembles that of the land vertebrates, and indicates the way, as shown in B, in which the typical skeleton of 

 the pectoral appendage of the tetrapoda has been derived from the biserial pectoral fin of fishes. (From specimen 

 in the authors collection from Scaumenac Bay. P. Q., Canada.) 



The pectoral fins enlarge and the girdle extends dorsally, uniting with the 

 occipital portion of the cranium. 



Within the pectoral fins, for the first time in the phylogeny of the vertebrates, 

 appears an axial skeleton that approaches, in the arrangement of its elements, 

 the characteristic structure of the appendages of the land vertebrates, i.e., Eus- 

 thenopteron. (Fig. 265.) Pelvic fins appear in the anal region. 



********* 



Thus the broad foundations for the evolution of the first air-breathing land 

 vertebrates is laid in the dipnoi and ganoids, the armored fishes of the upper 



