OF THE BRITISH ISLANDS. 107 



ORDER III. — Elasmobranchiata. 



Skeleton entirely cartilaginous ; tail unsymmetrical 

 (heterocercal), the upper lobe being the more produced ; 

 branchia attached to the skin by their outer margins with 

 usually several intervening gill-openings (deciduous external 

 gills developed in the embryo); the aortic bulb provided with 

 several series of valves ; ova large, few in number, impreg- 

 nated, and in many instances developed within a uterine 

 cavity. 



SUB-ORDER l.—Holocephala. 



Gill-opening single, covered by a fold of the skin ; the 

 maxillary and palatal apparatus coalescent with the skull ; 

 teeth few in number. 



' The Arctic Chimera, Rabbit-fish, or King of the Her- 

 rings, as it is popularly called (Chimcera monstrosd), No. 

 197, is the only representative of the Holocephalous division 

 of the Elasmobranch fishes taken in British waters, and 

 then upon very rare occasions, it being more strictly an 

 inhabitant of the deep and colder waters of the polar seas. 

 From the ordinary sharks, with which in many anatomical 

 points it closely agrees, it differs most essentially in the 

 possession of a single gill-opening, and in the character of 

 the dentition, the conspicuous teeth being but four in 

 number, two above and two beneath, their contour, in 

 addition to their number, much resembling the incisors of a 

 rabbit. A sharp and formidable spine arms the front 

 border of the first dorsal fin, and in the male a remarkable 

 erectile spiniferous appendage is developed from the front 

 of the head ; the upper lobe of the tail, which is very 

 long, tapers off gradually to the fineness of a thread. The 



