572 CHORD ATA. 



fer of the gill slits to the lower surface, the spiracles to the upper. The 

 teeth are usually pavement-like. The Pristidje, or sawfishes, are the mo.st 

 shark-like, but are readily recognized as belonging here by the position of 

 the gill slits. The common name is due to the fact that the snout is pro- 

 longed into a paddle-shaped blade, the edges armed with teeth. Pristh* 

 Raiid^; the typical members of the group ; Rata* Closely allied are the 

 Trygonid^, or sting rays, with whip-like tail with one or two spines, the 

 'stings' at the base; Daayatis* The torpedos (Torpedinid^) have 

 smootli skins, and have electrical organs, kidney-shaped bodies, on either 

 side between gill arches and pectoral skeleton. Torpedo* 



Order II. Holocephali. 



These forms, which luive uo conimuu English names, differ from 

 the selachii in having the pterygoqnadrate arch, which hears a few 

 large chisel teeth, fused with the cranium without a suspen.sor; in 



Fig. 600. — CliimcBra monstrosa. (From Kingsley.) 



having a dermal fold constituting an operculum, which covers the 

 gill slits; and correspionding with this, the gills more on the teleost 

 type (p. 5G6). Lastly, the vertebral centra are not developed. 

 Chinuera.* Fossils apjiear in the Devonian. 



The Cladoselachu (Cladoselache), Ichthtotomi {PJeuracanthus), and 

 AoANTHODmjE are paleozoic forms in which vertebral centra were lacking. 

 In Cladoselache the skeleton of the paired fln consisted of numerous simi- 

 lar radii and was more primitive than the archipterygium; Phiiracanthus 

 was diphycercal, and the head, as in Acanthodes, bore dermal bones. 



Sub Class II. Ganoidei. 

 The ganoids form a transition group in which elasmobranch 

 and teleost characters are mingled in a notable manner. They 

 have the spiral valve of the sharks, the swim bladder of the telosts; 

 the heart with the conns is selacliian, the respiratory structures — 

 the comb-like gills and the operculum — arc as distinctly teleostean. 

 The hyoid arch, with the development of the operculum, has not 

 entirely lost its respiratory function, since in garpike and sturgeon 

 it bears an opercular gill, and often there is a pseudolu'ancli in 

 the siiiraclc. The skeleton is always ossified in certain parts; large 



