2O4 



FISHES 



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- 

 lopterus, and Eutliynotus, retain in the adult a stage of vertebral 

 evolution which is closely paralleled by transitory stages in the 

 embryonic and young forms of Amia. 



Lepidosteus l is unique amongst existing Fishes in having 

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



and concave behind, 

 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- 

 c.n tion, of the noto- 



FIG. 118. A, two vertebrae from the trunk -region of chord, much in the 

 Lepidosteus; B, anterior face of a vertebra. c.n, , 



Anterior convex face of the centrum ; c.n', posterior Same Way as by 

 concave face ; h.a, parapophysis, with its articular facet the OTOWth of Ordi- 

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



fused supra-interdorsals ; i.s, radial element of the dorsal nary Centra. Later, 

 fin ;l.l. superior longitudinal ligament ; n.a, neural arch. r V,i q , n lirl ini nf 

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



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. Reference 

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

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

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



