SCALES OF FISHES. 593 



true bony structure, such as is shown in the two orders to be 

 next adverted to. In the ganoid scales, on the other hand, the 

 whole substance of the scale is composed of a substance which 

 is essentially bony in its nature; its intimate structure being 

 almost always comparable to that of one or other of the varieties 

 which present themselves in the bones of the Vertebrate skele- 

 ton ; and being very frequently identical with that of the bones 

 of the same fish, as is the case with the Lepidosteus (-Fig. 301), 

 one of the few existing representatives of this order, which, in 

 former ages of the Earth's history, comprehended a large num- 

 ber of important families. Their name (from ■rdvot: splendor) is 

 bestowed on account of the smoothness, hardness, and high 

 polish of the outer surface of the scales ; M'hich is due to the 

 presence of a peculiar layer that has been likened (though erro- 

 neously) to the enamel of teeth, and is now distinguished as 

 ganoin. The scales of this order are for the most part angular 

 in their form ; and are arranged in regular rows, the posterior 

 edges of each slightly overlapping the anterior ones of the next, 

 so as to form a very complete defensive armor to the body. 

 The scales of the placoid type, which characterizes the existing 

 Sharks and Rays, with their fossil analogues, are irregular in 

 their shape, and very commonly do not come into mutual con- 

 tact; but are separately imbedded in the skin, projecting from 

 its surface under various forms. In the Rays, each scale usually 

 consists of a flattened plate of a rounded shape, with a hard 

 spine projecting from its centre; in the Sharks (to which tribe 

 belongs the " dog-fish" of our own coast) the scales have more 

 of the shape of teeth. This resemblance is not confined to 

 external form; for their intimate structure strongly resembles 

 that of dentine, their dense substance being traversed by tubuli, 

 which extend from their centre to their circumference in minute 

 ramifications, without any trace of osseous lacunse. These 

 tooth-like scales are often so small, as to be iuvisible to the 

 naked eye ; but they are well seen by drying a piece of the skin 

 to which they are attached, and mounting it in Canada balsam ; 

 and they are most brilliantly shown by the assistance of polar- 

 ized light. A like structure is found to exist in the spiny rays 

 of the dorsal fin, which, also, are parts of the dermal skeleton ; 

 and these rays usually have a central cavity filled with medulla, 

 from which the tubuli radiate towards the circumference. This 

 structure is very well seen in thin sections of the fossil spiny 

 rays, which, with the teeth and scales, are often the sole relics of 

 the vast multitudes of Sharks that must have swarmed in the 

 ancient seas, their cartilaginous internal skeletons having entirely 

 decayed away. In making sections of bony scales, spiny rays, 

 &c., the method must be followed which has been already detailed 

 under the head of bone (§ 405). 



410. The seafes.of Eeptiles, \h& feathers of Birds, and the hairs, 

 hoofs, nails, claws, and horns (when not bony) of Mammals, are 



38 



