506 



CHORDATA. 



in the volk sac. In }[nsicliis and Carclmrias, as Aristollc know, there is the 

 formation of a ]ilacenta, which dillcrs from that ol ihc mammals in tluu the 

 embryonic blood su|iply arises from the blood-vessels of the yolk sac and are not 

 allantoic. There are oxiparous elasmobranchs, and in these the eg;; is sur- 

 rounded bv albumen and a shell, but these egLjs dilTer from tliose of liirds in that 

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

 threads for attachini; the cgt,' to plants, etc. 



Order I. Selachii. 



With the notoehord more or less completely replaced bv \-crtebral centra. 



Sub Order I. mi'LOSPONDYLI. Gill slits lateral, six or seven in num- 

 ber, a sint^'le dorsal fin. Cliliuiiyilosdacliiis with terminal mouth. Hcvaiiclnis,* 

 mouth normal, six gill slits; Hcplaiuinis, seven ;j;ill slits. 



Sub Order II. SQl^VLI (Euselachii). Normal sharks, with cvlindrical 

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



Fio. 556. ~R;/,; !:,,!;.■;. male, ventral view (after :\[obius and IlcinckeV B. 

 ventral, Br, iicelural fin; K. n>slruin; ,;, anus; r, copulatciry part of NeiUral; iw, gill elefls; 

 HI, mouth; 11, nostril; butwceii tlu-iu the oronasal frroove.' 



swimmers and rapaeimis, the teeth usually pointed, but in some the teeth arc 

 pavemenl-like. Xumerous families, dislingitished by vertebral characters, 

 number of dorsal lins, presence of nictitating membrane, etc. G.ALiom.E, 

 nictitating membrane present; besides the dog-sharks {^^llsU^hls* and Galins*)', 

 the largest sharks, Carrliariiiiis,-'' some with man-eating reputations The 

 hammer heads (/.v,i^un,V-) are closely allied. The mackerel sharks (LaiiiiKf'--) 

 and the great while ' mtm-ealer,' C\/n7;,/r,.,/e«,* lack nictitating membranes. 



