96 A HISTORY OF FISHES 



decrease in its strength, and those Bony Fishes which have 

 returned to the mail of their ancestors (although of a totally 

 different structure) may now be considered. As a general rule, 

 these are sluggish creatures, which have sacrificed speed and 

 agility and have come to depend on their armour for protection 

 from their enemies. The South American Cat-fishes, known as 

 Hassars or Cascaduras (Callichthys) , have the body completely 

 encased in mail, made up of a double row of broad over- 

 lapping shields on each side. The arrangement of the shields is 

 metameric, there being one pair to each vertebra or muscle 

 segment. AUied to the Hassars are the Mailed Cat-fishes or 

 Loricariids {Loricariidae) , a large and varied family confined to 

 the rivers of Central and South America. The body is pro- 

 tected above and on the sides by series of bony plates (Fig. 42c), 

 the chest and abdomen being either naked or covered with 

 much smaller plates. The plates on the sides have a metameric 

 arrangement, and may be more or less sharply keeled or 

 variously armed with small spines set in sockets (Fig. 42c). 

 These smaller spines are of special interest, for when examined 

 microscopically they are seen to be formed of dentine capped 

 with enamel; thus, they are essentially similar in structure to 

 the dermal denticles of the Selachians, and this represents 

 another of the rare examples of the course of evolution having 

 been reversed. The Mailed Cat-fishes are sluggish creatures, 

 spending most of their time attached to stones or other objects 

 at the bottom of a stream, and the bony armour provides them 

 with an efficient protection against enemies. In some very 

 closely related fishes from the mountain streams of the Andes 

 (Cyclopiiim), where the absence of carnivorous fishes renders 

 armour superfluous, the scutes have disappeared and the skin 

 is quite naked (Fig. 93). Among other Bony Fishes with the 

 body more or less completely cuirassed with bony shields, the 

 Sea Robins of deep water {Peristedion), and the curious little 

 Pogge or Bullhead (Agonus) found round our own coasts 

 (Fig. 42b), may be mentioned. 



Among the members 'of the large and diverse order of fishes 

 known as Tube-mouths {Solenichthyes) there occur several 

 interesting modifications of the scaly covering. In the Snipe- 

 fishes (Macrorhamphosus) , for example, each scale consists of 

 a bony basal plate, rhomboid in shape, which is produced 

 into a curved and backwardly directed spine, containing a 

 definite pulp-cavity, reminiscent of that of the Selachian 

 denticle. In the remarkable Shrimp-fishes [Centriscidae) the 



