234 



DIPNOI 



fins and a diphycercal tail. The Dipnoi may well have been derived 

 from some earlier form of similar shape (Fig 227). 



It is important to notice that the fin -supports, radials or 

 somactids, are segmented and intimately connected with the axial 

 skeleton (Fig. 205). The radials and neural spines together form 

 continuous three-jointed ossified rods. When the dorsal and anal 

 fins are separate, the proximal segments may be concentrated 

 and fused (Scaumenada, A. Smith Woodward [503]). This tendency 

 is also seen in the anal region of the continuous fin of Ceratodus* 



That the ventral caudal lobe in modern Dipnoi is supported by 

 separate radials, and not by rigid unjointed haemal spines, would 



B 



nc. 



Fio. 204. 



A, transverse section of vertebral column of a young Protopterus, showing the invasion of 

 the fibrous notochordal sheath by the menoblastic cells. B, left-side view of a portion of the 

 vertebral column (abdominal region) of Ceratodm Forsterl, Krefft., of which the anterior half 

 has been cut longitudinally. C, view of the same cut across. 6d, basidorsal cartilage ; 6i', 

 basiventral cartilage ; c,1, canal for ligament ; e, elastica externa ; f.sh, fibrous sheath ; h.a, 

 haemal arch (Itasiventral) ; h.c, haemal canal ; n.a, neural arch ; n.c, nerve-cord ; n.cl, muni 

 canal ; n.sp, neural spine ; nt, notochord ; p.r, pleural rib. 



seem to be due to the genuine diphycercal character of the tail, 

 and not, as Balfour supposed [30], to the loss of the caudal fin 

 itself. This character indicates a primitive structure (p. 101), and 

 weighs against Dollo's theory. 



The notochord extends to near the pituitary region ; there is' 

 no distinct joint between the skull and the vertebral column, which 

 pass gradually into each other. Several vestigial vertebral segments 

 seem to be incorporated into the hind part of the skull behind the 

 vagus nerve. In Ceratodus, besides the hypoglossal roots, two 

 complete spinal nerves pass through this region ; three neural 

 arches and spines are found attached to it above, and a pair of 

 enlarged 'cranial ribs ' articulate to it at the sides (Figs. 206, 209). 

 Such cranial ribs are found in all living Dipnoi, and even in such 



