Si8 PISCES— FISHES. 



ACANTHODEI. 



Another interesting but quite extinct group, whose position was for 

 long a matter of dispute, but which is now usually placed near 

 Elasmobranchii, is that of the Acanthodei. These flourished principally 

 in Devonian times, but lived on also through the Carboniferous to the 

 Lower Permian. They are usually rather small fishes, with minute 

 rhomboidal shagreen-like scales, and a strong spine in front of each 

 fin, except the caudal. In some genera (Parexus, Climatius) there are 

 two rows of small intermediate spines between the proper pectorals and 

 the ventrals. 



Ganoidei. 



This ancient " order " of armoured fishes flourished in 

 Devonian and Carboniferous ages, but is now represented 



Fig. 222. — Sturgeon (Acipemer sturio) from side. 



Note the elongated snout, the barbules bounding the ventral 

 mouth, the operculum covering the gills, the rows of bony 

 scutes, the markedly heterocercal tail. 



by only seven genera, of which the sturgeon {Acipenser) 

 and the bony pike (Lepidosteus) are the most familiar. 



The skin bears large scales or bony scutes. The tail is 

 either heterocercal or homocercal. Membrane bones invest 

 the skull and shoulder-girdle. The endoskeleton is in great 

 part cartilaginous in Acipenser, Scaphirhynchus, and Polyo- 

 don, but is ossified in Lepidosteus, Polypterus, Calamoichthys, 

 and Atnia. In the first three the notochord is uncon- 

 stricted ; in the others there are distinct vertebral bodies, 

 — opisthoccelous in Lepidosteus, amphicoelous in the other 

 three genera. The fore-brain has a non-nervous roof. 

 There is a spiral valve in the intestine, but it is very small 

 in Lepidosteus. The food canal ends apart from and in 

 front of the urogenital aperture. There are also abdo- 

 minal pores. An air-bladder is present with a persistent 

 open duct. The openings of the gill-clefts are covered 



