THE SKELETON IN FISHES. SCALES AND FIN-RAYS. 105 



Acipenser a single row extends along the middle of the back 

 and two along each side. 



The majority of Teleosteans have thin flattened scales 

 which differ from those of Ganoids in being entirely meso- 

 dermal in origin, containing no enamel. There are two prin- 

 cipal types of Teleostean scales, the cycloid and ctenoid. A 

 cycloid scale is a flat thin scale with concentric markings 

 and an entire posterior margin. A ctenoid scale differs in 

 having its posterior margin pectinate. The Dipnoi have over- 

 lapping cycloid scales. The rounded scales of Amia and of many 

 fossil ganoids such as Holoptychius are shaped like cycloid 

 scales, but differ from them in being more or less coated with 

 enamel. In Eels and some other Teleosteans the scales are 

 completely degenerate and have almost disappeared. Some 

 Teleosteans, like Diodon hystrix, have scales with triradiate 

 roots from which arise long sharp spines directed backwards. 

 These scales, which resemble teeth, contain no enamel ; they 

 become erect when the fish inflates its body into a globular 

 form. Many Siluroids have dermal armour in the form of 

 large bony plates which are confined to the anterior part 

 of the body. In Ostracion the whole body is covered by 

 hexagonal plates, closely united together. 



The fin-rays are structures of dermal origin which entirely 

 or partially support the unpaired fins, and assist the bony 

 or cartilaginous endoskeleton in the support of the paired fins. 



In Elasmobranchs, Dipnoi, and Chondrosteous ganoids the 

 skeletons of the fins are, as a rule, about half of exoskeletal, 

 half of endoskeletal origin, the proximal and inner portion 

 being cartilaginous and endoskeletal, the distal and outer portion 

 being exoskeletal, and consisting of horny or of more or less 

 calcified fin-rays. In bony Ganoids and Teleosteans the endo- 

 skeletal parts are greatly reduced and the fins come to consist 

 mainly of the fin-rays, which are ossified and frequently become 

 flattened at their distal ends. 



The fin-rays of the ventral part of the caudal fin are carried 



