Plagiostomi — Sharks and Rays. 3 



highest stage of specialization or elaboration to which the piscine 

 class has attained. 



In tracing the fishes through the successive ages of the past, 

 it is interesting to note the close correspondence between the 

 history of the race and the history of an individual modern Tele- 

 ostean, at least in one point — the structure of the tail. All the 

 older members of the class either had the extremity of the body 

 straight and tapering, with the fin equally developed above and 

 below (Fig. 5), or there was a slight upward bend of the vertebral 

 column, with the lower lobe of the tail-fin much larger than the 

 upper (Fig. 6). In later fishes, the upturned end of the body in 

 the unequally-lobed tail has become more and more abbreviated, 

 and the rays of the fin have gradually become so disposed that to 

 all external appearance the tail has assumed perfect symmetry 

 (Fig. 7). Such changes are precisely repeated in the embryonic 

 history of a living Teleostean, in which the tail is first pointed, 

 then upturned, and then externally symmetrical. 



FORMS OF TAILS OF FISHES. 



Fig. 5. — Diphycercal.* 

 Primitive form. 



Fig. 6. — Heterocercal.t 

 Ancient form. 



Fig. 7. — Homocercal.t 

 Modern form. 



Oeder I.— CYCLOSTOMATA. 



!No undoubted fossil representatives have hitherto been recognized. 



Oeder II.— PLAGIOSTOMI. 



In these fishes the skeleton remains cartilaginous throughout "Wall-cases, 

 life. The cranium itself is not divided into any distinct tracts ** os - * t0 3 - 

 by sutures or ossifications, and the two foremost of the "visceral 

 arches" (cartilaginous rods in the walls of the alimentary tube), Table-cases, 

 which are modified as jaws and hyoid cartilages, have a very Nos - 25 10 33- 

 slight connection with it. The jaws are mainly suspended by the 

 upper element of the hyoid arch (the " suspensorium ") and by a 

 ligament in front ; or there is sometimes {e.g. in Cestracion and 

 Noticlanus) direct contact either behind or in advance of the eye 

 (see Fig. 17). The axial skeleton of the trunk varies from a 

 primitive persistent notochord to a well-calcified vertebral column, 



* Diphycercal, double tail-fin. f Heterocercal, unequal-lobed tail-fin. 



X Hoatocercal, equal-lobed tail-fin. 



