fr 



SKELETON 



57 



metrical appearance. In the homocercal tail, which occurs in Amia 

 and all teleosts since the cretaceous^ there is the same upward bend 

 to the vertebral column (fig. 52, A), but symmetry is restored 

 externally by the reduction of the neural arches and the development 

 and fusion of the haemals into larger plates (hypurals), while the 

 lower lobe of the tail grows out to equal the other (fig. 52, D). 



:'}. 52.— Tails of fishes. A, young Amia, skeleton; B, diphycercal; C, heterocerca; 

 D, homocercal; h, hypurals; n, notochord; s, spinal cord 



CYCLOSTOMES have a persistent notochord, which increases in size with 

 the growth of the animal, and lacks constrictions, since no centra are developed. 



Fig. S3. l-io. 54. 



Fig. S3. — Diagrammatic sections of elasmobranch vertebrae. A, B, cyclospondylous; 



C, asterospondylous. 

 Fig. S4' — Cross-section of teleost vertebra; bone, black; cartilage, dotted. 



In the myxinoids there are cartilaginous neurapophyses and intercalaria devel- 

 oped in the caudal region; in the lampreys they occur in the trunk as well. 



FISHES.— In all fishes only two vertebral regions— trunk and tail— are 

 differentiated. In the elasmobranches the typical vertebra: are developed in 

 cartilage, with intercalaria in connexion with the arches. Usually the centra 



