CHAPTER VIII. 



GENERAL ACCOUNT OF THE SKELETON IN 

 FISHES 1 . 



EXOSKELETON. 



The most primitive type of exoskeleton is that found in 

 Elasmobranchs and formed of placoid scales; these are tooth- 

 like structures consisting of dentine and bone capped with 

 enamel, and have been already described (p. 4). In most 

 Elasmobranchs they are small and their distribution is fairly 

 uniform, but in the Thornback skate, Raia clavata, they have 

 the form of larger, more scattered spines. In adult Holo- 

 cephali and in Polyodon and Torpedo there is no exoskeleton, 

 in young Holocephali, however, there are a few small dorsal 

 ossifications. 



The plates or scales of many Ganoids may have been 

 formed by the gradual fusion of elements similar to these 

 placoid scales, and often bear a number of little tooth-like 

 processes. In Lepidosteus, Polypterus, and .many extinct 

 species, these ganoid scales, which are rhomboidal in form 

 and united to one another by a peg and socket articulation, 

 enclose the body in a complete armour. In Trissolepis part of 

 the tail is covered by rhomboidal scales, while rounded scales 

 cover the trunk and remainder of the tail. Acipenser and 

 Scaphirhynchus have large dermal bony plates which are not 

 rhomboidal in shape and do not cover the whole body. In 



1 The following general works on fishes may be referred to : Bashford 

 Dean, Fishes, Living and Fossil, New York, 1895. A. Giinther, An 

 Introduction to the Study of Fishes, Edinburgh, 1880. A. A. W. Hubrecht 

 and M. Sagemehl, Fische in Bronn's Classen und Ordnungen des Thier- 

 reichs, Band vi. Leipzig, 1876. 



