376 A HISTORY OF FISHES 



fishes or Rouscttcs of the family Scyliorhinidae \ and the family 

 Carcharinidae, including the Blue Sharks {Carchari?ius, etc.), Topes 

 {Eugaleus), Hounds [Muslelus) and Hammer-headed Sharks 

 {Sphyrna). The last sub-order, the Squaloidea, includes four 

 families which have sprung from the same common ancestral 

 stock, and have subsequently evolved along divergent lines, as 

 well as three more specialised families. In all the members 

 of this sub-order, either the two dorsal fins are each preceded 

 by a spine or the anal fin is absent (Figs. 53B; 6oa). Two of 

 the four families mentioned above [Cochliodontidae, Hybodontidae) 

 are extinct; the other two are the Bull-headed Sharks (Hetero- 

 dontidae) and the family Squalidae, including the Spiny Dog- 

 fishes (Squalus)^ Bramble Shark (Echinorhi?ius) , Greenland Shark 

 (Somniosus) and a number of other forms. This family has given 

 rise to the remarkable bottom-living Angel-fishes or Monk-fishes 

 of the family Squatinidae, and the specialised Saw Sharks {Pristio- 

 phoridae) have probably been derived from the extinct Proto- 

 spinacidae. 



The Rays {Hypotremata) may be divided into two sub-orders, 

 the first or Narcobatoidea, including only the Electric Rays or 

 Torpedoes [Narcobatidae) . Among the more primitive families 

 of the Batoidea, the second sub-order, are the Guitar-fishes 

 {Rhino batidae) and Saw-fishes (Pristidae)^ which have retained 

 the elongate form and muscular tail of the Sharks (Fig. 38), and 

 the other families include the true Skates and Rays {Raiidae), 

 Sting Rays (Trygonidae) , Eagle Rays {Myliobatidae) , and Devil- 

 fishes (Mobulidae). 



The last sub-class of Selachians, the Holocephali, includes the 

 few existing Chimaeras and a host of extinct forms. The 

 members of this group are readily recognised by having the 

 primary upper jaw (palatoquadrate cartilage) fused with the 

 cranium (Fig. 56A), and by the gill-clefts opening into a 

 chamber with a single external aperture, a character in which 

 they approach the Bony Fishes (Fig. 140). 



In the class of Bony Fishes {Pisces) the genera and species are 

 so much more numerous and diverse that their arrangement 

 becomes a matter of some difficulty, and there is considerable 

 diversity of opinion as to the division into sub-classes and orders. 

 The classification outlined here is mainly that of Dr. Regan, 

 and in broad outline may be accepted as one which would 

 receive the support of a large number of modern ichthyologists. 

 Three sub-classes are recognised : Palaeopterygii, Crossopterygii and 

 Neopterygii. The first two of these contain very few existing 



