Pisces. 



79 



of Scaumenac Bay, P.Q., Canada, but its tail has never been 

 found, being probably sdaleless. Asferolepis is an allied genus. 



Class 5.— PISCES. 



The true fishes all possess a lower jaw and complete or 

 rudimentary paired fins. They are most commonly divided 

 into the subclasses or orders of Elasmobranchii (sharks, rays, 

 and chimgeras), Dipnoi (mud-fishes), Ganoidei (enamelled- 

 scaled fishes), and Teleostei (bony fishes). This arrangement, 

 however, was originally based chiefly on a study of the fishes 

 now existing ; and more recent investigations among extinct 

 fishes have shown that it cannot be maintained. The limits of 

 these groups are indicated by the brackets to the right of the 

 table on the next page, which explains the system of classifica- 

 tion adopted in the Collection. 



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 tin equally developed above and 

 below (Fig. 105), or there was a slight upward bend of the verte- 



FOEMS OF TAILS OF FISHES. 



Fig. 105. — Diphycercal. 

 Primitive form. 



Fig. 106. — Heterocercal. 

 Ancient Form. 



Fig. 107.— Homocercal. 

 Modern form. 



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

 than the upper (Fig. 106). 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 per- 

 fect symmetry (Fig. 107). 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 sym- 

 metrical. 



