572 CHORD ATA. 



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

 teeth are usually pavement-like. The PRISTINE, or sawfishes, are the most 

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

 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. Pristis* 

 RAIID^E; the typical members of the group ; Raia.* Closely allied are the 

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

 ' stings,' at the base ; Vasyatis* The torpedos (TORPEDINID^E) have 

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

 side between gill arches and pectoral skeleton. Torpedo* 



Order II. Holocephali. 



These forms, which have no common English names, differ from 

 the selachii in having the pterygoquadrate arch, which bears a few 

 large chisel teeth, fused with the cranium without a suspensor; in 



FIG. 600. ChimoRra monstrosa. (From Kingsley.) 



having a dermal fold constituting an operculum, which covers the 

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

 type (p. 566). Lastly, the vertebral centra are not developed. 

 Chimcera.* Fossils appear in the Devonian. 



The CLADOSELACHII (Cladoselache), ICHTHYOTOMI (Pleur acanthus), and 

 ACANTHODID^E are paleozoic forms in which vertebral centra were lacking. 

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

 lar radii and was more primitive than the archipterygium; Pleuracanthus 

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



Sub Class //, 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 conus is selachian, the respiratory structures 

 the comb-like gills and the operculum are 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 pseudobranch in 

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



