VEfiTEBRATA. 189 



SELACHOIDF.I (Sharks). Gill-openings mostly fine, lateral. No 

 gill-covers. An anal fin. Body fusiform. Notochord not per- 

 sistent. Ovoviviparous. 



The pectoral fin is free. In some species there are two spi- 

 racles on the top of the head communicating with the pharynx. 

 The teeth are mostly in several rows, compressed, triangular, and 

 sometimes trilobed, with their edges serrated ; they are united 

 by ligament to the jaws. 



A foetal peculiarity is the existence of external filamentary 

 branching, as in the Batrachia ; they are early removed by 

 absorption. 



Some of these fish attain to a large size Carcharodon Ronde- 

 letii 40 feet ; Selache maxima 30 feet ; Carcharias vulgaris 25 

 feet [not found in the English seas] ; Alopias vulpes 13 feet ; 

 Lemma cornubica 9 feet. The blue shark ( Carcharias glaucus), 

 8 or 9 feet, is sometimes caught on the Cornish coast. They are 

 nearly all very bold and voracious. 



Squatimda. CestraciontidcB. EMnodontidce. 



Squatina=Khina Cestracion. Ehinodon. 



(Angel-fish). *Acrodus. 



. 7 . , *Ptychodus. Lamnidce. 



Pnstophand*. gdache 



Pristiophorus. Hybodxmtid*, Carcharodon. 



Spinacidce. *Hybodus. Alopias (Thresher). 



Echinorhinus. Lamna (Porbeagle). 



CentrophoruB. ScyMd*. 



Scymnus. Scylhum (Dog-fish). Carchariidce. 



Lsemargus. Pristiurus. Carcharias. 



Spinax. Mustelus (Hound). 



Acanthias. Notidanidce. Zygna=Sphyrna. 



Centrina. Notidanus. Galeus (Tope). 



Order VII. HOLOCEPHALI. 



Notochord persistent, skeleton cartilaginous; skull without 

 sutures. One gill-aperture, with a rudimentary gill-cover and 

 membrane. Skin naked. 



The rudimentary gill-cover is concealed by the skin. Although 

 there are five openings in the gill-cavity there is only one external 

 opening. The upper jaw is united to the skull ; and, in lieu of 

 teeth, there are four hard plates above and two below. 



