356 ZOOLOGY 



and the single dorsal fin placed far back over 

 the base of the tail. The Pceciliidse are rela- 

 tively small, often spotted or striped, with the 

 single dorsal fin nearly always posterior to the 

 middle, and the caudal (tail) fin rounded or 

 squared (truncate), not bifurcated. The scales 

 are cycloid, with strong basal radii. 



Spiny-rayed (<?) AcanthopterygH. Spiny-rayed fishes; generally 



known by the anterior position of the pelvic 

 fins, and the rays of some of the fins hard and 

 spinelike. The scales are generally ctenoid, 

 with not merely marginal teeth, but a consid- 

 erable area covered with fine projections or 

 variously modified. Such scales usually have 

 strong basal radii, arranged in a fanlike manner, 

 and the basal margin is likely to be scalloped. 

 When a typical spiny-rayed fish, of which the 

 perch or the sea bass may be taken as an 

 example, has been thus defined, it is necessary 

 to state that within the group as now recog- 

 nized are many exceptions. There are, 'first of 

 all, the relatively primitive families, such as 

 the flying-fish group, in which the scales are 

 wholly cycloid. These strange fishes have the 

 pectoral fins enormously enlarged, serving as 

 organs of temporary flight; and the lower lobe 

 of the tail fin is elongated, so that it may be 

 used to strike the water as the flying fish ap- 

 proaches its surface, and thus give it a new 

 start. These fishes are of course specialized 

 animals in their own way, but not in the direc- 

 tion of the mass of the acanthopterygians, in 

 relation to the special characters in which they 

 are primitive. Aside from the relatively primi- 



