384 



Vertebrata. 



some of the remaining placoid scales form large spines ; and that the buccal teeth 

 are low knobs (sometimes pointed) or plates, which are aiTanged in several rows 

 and form a mosaic over the edges of the jaws. In general appearance, therefore, 

 the Skates differ considerably from the Sharks. The skate type is not always 

 developed to the same extent ; in some forms the pectoral fins are smaller, the tail 

 more powerful ; whilst, on the other hand, there are Sharks (Squatina, the Sea- 

 angel), which are somewhat flattened, with the eyes turned upwards, and large 

 horizontal pectoral fins reaching antero-posteriorly along the sides of the head, but 

 not attached to it. There is indeed a complete series of transitional forms, between 

 the usually slim shark type, to the most extreme ray type with its discoid shape, 

 wider than it is long, and with its thin caudal whip. In British seas there 

 are several species (chiefly of the genus Baja), all typical Rays. Of forms 

 belonging to the Southern seas may be mentioned the Electric Skates 

 (Torpedo), and the Saw-fish (Fristis) ; the former are well known on account 

 of their powerful electric organs, which lie on either side of the head ; in the 

 Sawfish the snout is drawn out into a long, straight, naiTOw plate, with a series of 

 large, laterally directed, teeth on each edge. Both the Electric Skates and the 

 Saw-fish, but especially the latter, belong to the more shark-like Rays, with toler- 

 ably powerful tail. Both genera occur in the Mediterranean. 



3. The Cat-fishes (Holocephali) genus Chimssra, etc., form a small 

 division of the Selachians, which differ from their allies, and approach the 

 following orders chiefly in the possession of an operculum (which is, how- 

 ever, not supported by skeletal plates) ; the giU lameUas completely cover the side 

 -of the septum, but do not project over its outer rim (Kg. 309 B). The skin is 



Fig. 314. ChimoBra monstrosa, ^ . 



■for the most part naked, the mouth armed with a small number of large teeth. 

 The upper portions of the mandibular and hyoid arches are attached to the skull. 

 In other respects they exhibit for the most part the characters of other Selachians. 

 One species, Ch. monstrosa, is abiindant in the Mediten-anean, on the coast 

 of Noi-way, and elsewhere. 



Order 3. Ganoidei. 



The skeleton consists of cartilage and bone ; conus arteriosus and 

 spiral valve are present ; an operculum supported by bony plates ; 

 -often a spiracle ; swim-bladder or true lung. The skin usually 



