506 



CHORDATA. 



in the yolk sac. In Ifiistelus and Carcharias, as Aristotle knew, there is the 

 formation of a placenta, which differs from that of the mammals in that the 

 embryonic blood supply arises from the blood-vessels of the yolk sac and are not 

 allantoic. There are oviparous elasmobranchs, and in these the egg is sur- 

 nnmclfd by albumen and a shell, but these eggs differ from those of birds in that 

 the shell is horny and is usually drawn out at the four corners, sometimes with 

 threads for attaching the egg to plants, etc. 



Order I. Selachii. 



With the notochord more or less completely replaced by vertebral centra. 



Sub Order I. DIPLOSPONDYLI. Gill slits lateral, six or seven in num- 

 ber, a single dorsal fin. Chlamydoselaclnis with terminal mouth. Hexanchus* 

 mouth normal, six gill slits; Heptanchus, seven gill slits. 



Sub Order II. SQUALI (Euselachii). Normal sharks, with cylindrical 

 bodies, free thoracic fins, heterocercal tail, lateral gill slits. Most are fast 



n 



Fir j- 556 Rai" balis, male, ventral view (after Mobius and Heincke). B 

 tral, Br, pectoral im; R, rostrum; a, anus; c, copulatory part of ventral; ks gill clefts- 

 in, mouth; n, nostril; between them the oronasal groove. 



wimmers and rapacious, the teeth usually pointed, but in some the teeth are 

 Numerous families, distinguished by vertebral characters, 

 imber of dorsal fins, presence of nictitating membrane, etc. GALEIDE 

 nig membrane present; besides the dog-sharks (Miistelus* and Calais*) 

 largest sharks, Carckarinus* some with man-eating reputations The 

 ler heads (Zygana*) are closely allied. The mackerel sharks (Lamna*) 

 vhite 'man-eater,' Carcharodon* lack nictitating membranes. 



