IV. VERTEBRATA: PISCES, GANOIDEI. .507 



The foregoing are asterospondylous. The dog-fishes {Acanthias vulgaris* 

 fig. SSS) a-re cyclospondylous; spine in front of each dorsal fin. 



Sub Order III. RALfE, skates; body flattened horizontally (fig. 556); the 

 pectoral fins, also flattened, united to sides of body, the union usually extending 

 to tip of snout, and frequently baclc to the pelvis, giving the body a rhombic 

 appearance from above. The animals swim by undulating motions of these 

 fins. The union of fins to the side has resulted in transfer of the gill slits to the 

 lower surface, the spiracles to the upper. The teeth are usually pavementdike. 

 Pristid.e, sawfishes, snout prolonged, the edges with teeth. Pristis* Raiid.e; 

 the typical members of the group; Raia* Trygonid.«:, sting rays, whipdike 

 tail with one or two spines at base; Dasyatis* Torpedinid^, smooth skins, 

 electrical organs between gill arches and pectoral skeleton. Torpedo* 



Order II. Holocephali. 



These forms, which have no common names, differ from the selachii in 

 having the pterygocjuadrate arch, which bears a few large chisel teeth, fused 

 ■with the cranium; in having a. dermal fold (operculum) which covers the gill 



Fig. 557. — Chim(Era monstrosa (from Kingsley). 



slits; and corresponding with this, the gills more on the teleost type (p. 501). 

 Lastly, the vertebral centra are not developed. Chimara* Fossils appear in 

 the Devonian. 



Cladoselachii, Ichthyotomi {Pleiiracantkus), and Acanthodid^, paleo- 

 zoic; vertebral centra lacking. 



Sub Class II. Ganoidei. 



The ganoids form a transition group in which elasmobranch and 

 teleost characters are mingled. They have the spiral valve of sharks, 

 the swim bladder of teleosts; the heart with conus is selachian, the respira- 

 tory structures — the comb-like gills and the operculum — are as distinctly 

 teleostean. With the development of the operculum the hyoid arch has 

 not entirely lost its respiratory function, since in garpike and sturgeon 

 it bears an opercular gill, and often there is a pseudobranch in the spiracle. 

 The skeleton is always ossified in certain parts; large membrane bones 

 lie on the shoulder girdle, on the roof and floor of the skull (parasphenoid) ; 

 the horny threads of the fins are bony rays. In general the skeleton ranges 

 between two extremes — an extremely primitive cartilaginous condition 

 with persistent notochord, and one with considerable ossification, Lepidos- 



