288 CHARACTERS OF VERTEBRATE ANIMALS 



in an ordinary flat -fish, the breadth being increased by the 

 enormous development of the pectoral fins, which form great 

 wings extending from the head to the pelvic fins. As a result 

 of this depressed shape, the gill-slits, of which the typical five 

 pairs are present, are situated on the under side. The tail is 

 narrow and forms a mere appendage, upon the upper side of 

 which the small dorsal fins are placed, while the anal fin is absent. 

 There are six families, of which five may be specially mentioned. 



1. True Rays. The numerous species of the rhomboidal- 

 shaped fishes belonging to this family have a wide distribution, 

 but are chiefly temperate forms more abundant in the northern 

 than in the southern hemisphere. The numerous small teeth 

 are closely packed in a considerable number of rows, arranged 

 so as to form two roller-like surfaces which bite against one 

 another. There are about twelve British species, of which the 

 commonest are the Skate (Raia batis) y in which the skin is com- 

 paratively smooth, and the Thornback (Raia clavata), which has 

 the dorsal surface irregularly studded with large placoid scales 

 of curious shape, each consisting of a circular disc from which a 

 thorn-like spine projects, while a row of similar scales runs down 

 the middle of the back. 



2. Saw-Fishes. This includes five species of fishes especially 

 characteristic of tropical regions, and one of which (Pristis 

 antiquorum) is common in the Atlantic and Mediterranean. The 

 body is not broadened out to the same extent as in Rays, and 

 in this respect is, so to speak, half-way between them and the 

 Sharks. A remarkable peculiarity is found in the prolongation 

 of the snout into a long flattened rostrum, in the sides of which 

 are imbedded sharp teeth. These fishes may exceed 20 feet in 

 length, as much as six of this being taken up by the "saw". 



3. Eagle -Rays. These are found both in tropical and tem- 

 perate zones, one species, the Whip -Ray (Myliobatis aquila] 

 being occasionally taken in British seas. The extremely slender 

 tail is armed above with a strong spine, while the teeth, instead 

 of being pointed, have flattened crowns, and are in contact with 

 one another at the edges, so as to constitute a very perfect 

 crushing surface, shaped like a roller. 



4. Sting- Rays. This group includes much -modified Rays, 

 commonest in the tropics. The body is excessively broad, and 

 the pectoral fins run forwards so as to surround the front of the 



