10 THE PLAGIOSTOMIA. 



PLAGIOSTOMIA. 



The Plagiostomes, Sharks, Skates, and Rays, are distinguished from the 

 Chismopnea by the presence of from five to seven external gill openings and the 

 absence of an erectile dorsal spine, and, in the males, by the lack of a frontal 

 tenaculum and of subventral tenacula. 

 The Plagiostomia are divided into two suborders : — 



Body subfusiform; pectorals not attached to the head 



gill openings at the side of the body . . . Antacea (page 10) 



Body discoid; pectorals attached to the head 



gill openings below the disk Platosomia (page 257) 



ANTACEA. 



Body fusiform to subcylindrical. Pectoral fins free from the head, with 

 short basal articulations and comparatively few radials; shoulder girdle not 

 directly or firmly attached to the vertebral column. Dorsal fins one or two, 

 without or with a fixed spine in front. Anal fin present or absent. Nostrils 

 separated from the mouth or connected with it by grooves. Eyes lateral, 

 without or with a nictitating membrane or fold. Spiracles present or absent. 

 Mouth inferior or anterior; teeth numerous. Five to seven gill openings, in 

 front of the pectorals or above them. Hemispheres of the brain not distant from 

 the optic lobes. Name from "Avra & d/a's. 



Synopsis of Groups of Families. 



Body subcylindrical 



one dorsal, without a fixed spine; an anal fin; caudal axis low 

 gill openings six to seven ; no nictitating membrane 



spiracles present .... Hexanchoidei (page 11) 



two dorsals, without spine; an anal fin 

 gill openings five; caudal axis low 



spiracles present; no nictitating membrane 



Carcharoidei (page 11) 

 gill openings five; caudal deep, axis raised 



spiracles present, rarely absent; no nictitating membrane 



nasoral grooves none . . . Isuroidei (page 12) 



two dorsals (except on Pentanchus) , without spine; an anal 



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