50 COMPARATIVE ANATOMY 



basidorsals and interdorsals, and thus may be twice as numerous 

 as the centra (Fig. 39). l The lower arches consist of basivcntral 

 cartilages, between which are sometimes intercalated a series of 

 interventrals (Fig. 35), and which, in the tail, are produced into 

 Jicemal spines: these may be formed of distinct infravcntral 

 elements. 



In Dipnoans the interdorsals and interventrals are fused or 

 wanting, and in Bony Ganoids and Teleosts the various elements 

 usually become united in the fully-formed vertebra. Distinct fused 



fcr 



n.a 



n.a, 



n.sp 



n.a 



h.a 



FIG. 39. PORTIONS OF THE VERTEBRAL COLUMN OF Scyllium canicida. 

 (From Parker's Practical Zooloyy.) 



A and B, from the trunk ; G and D, from the middle of the tail ; A and C, two 

 vertebr<B in longitudinal section ; B and D, single vertebrae viewed from one 

 end ; 6, calcified portion of centrum ; c, centrum ; for, foramen for dorsal, 

 and for', for ventral root of spinal nerve; h.a, haemal arch (basi- ventral) ; 

 h.c, haemal canal; h.sp, haemal spine; i.n.p, intercalary piece (interdorsal, 

 or interneural plate) ; n.a, neural arch ; n.c, neural canal; n.p, neural plate 

 (basi-dorsal) ; n.sp, neural spine ; ntc, intervertebral substance (remains of 

 notochord) ; r, proximal portion of rib ; tr.pr, transverse process (basal 

 stump). 



pairs of supradorsals, however, persist and remain unossified in 

 Lepidosteus (Fig. 42), and in the caudal region of Amia the basi- 

 dorsals and basiventrals remain separate from the interdorsals and 

 interventrals, thus giving rise to double vertebral bodies (pre- 

 centra, bearing the arches, and archless postcentra). A somewhat 

 similar condition is seen in the Jurassic Eurycorams and other 

 fossil Ganoids. From what has been said above, it will be seen 

 that the number of arch elements does not necessarily correspond 



1 It was mentioned on p. 48 that in the Lamprey there are two pairs of 

 arcualia to each myotome : it is possible that they correspond to alternating 

 basidorsals and interdorsals. 



