204 



FISHES 



CHAP. 



down-growing, archless, dorsal half-rings, structures comparable 

 to post-centra are produced. In brief, Eurycormus, as well as 

 such other extinct Amioid genera as Caturus (Fig. 117, A), Cal- 

 loptems, and Euthynotus, retain in the adult a stage of vertebral 

 evolution which is closely paralleled by transitory stages in the 

 embryonic and young forms of Amia. 



Le-pidosteus ^ is unique amongst existing Fishes in having 

 opisthocoelous vertebrae ; that is, the centra are convex in front 



and concave behind, 



is. . i 



I.e. A 



—^ •' ^ 



and therefore articu- 

 late with one an- 

 other by ball-and- 

 socket joints (Fig. 

 118). This condi- 

 tion is due to the 

 development of a 

 series of interver- 

 tebral rings of car- 

 tilage round the 

 notochord. The 

 subsequent inward 

 growth of each of 

 these rings leads to 

 the constriction, and 

 ultimately to the 

 complete oblitera- 

 tion, of the noto- 



'--k.er- 



Pig 118 -A, two vertebrae from the tnmk-region of chord, much in the 

 l^epulosteus ; B, anterior face of a vertebra. 

 Anterior convex face of the centrum ; 

 concave face ; h.a, parapophysis, with its articular facet 

 for a rib ; i.c, median cartilage, representing a pair of 

 fused supra-interdorsalg ; i.s, radial element of the dorsal 

 fin ; l.l, superior longitudinal ligament ; n.a, neural arch. 

 (From Wiedersheim, after Balfour and W. N. Parker.) 



posterior Same Way as by 

 the growth of ordi- 

 nary centra. Later, 

 this solid mass of 

 cartilage becomes 

 transversely divided by a cleft which is convex anteriorly and 

 concave behind (Fig. 116, C), and of the two portions one fuses 

 and co-ossifies with the centrum of the vertebra in front, and the 

 other with the one pertaining to the vertebra behind. Eeference 

 to Fig. 116 will show that the grouping of the vertebral elements 

 to form the individual vertebrae is not the same as in Amia. 



' F. M. Balfour and W. N. Parker, Phil. Trans. 173, 1882, p. 388. 



