VERTEBRATED ANIMALS. 263 



the true Bony structure, such as is shown in the two Orders to be next 

 adverted-to. 



659. 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 always comparable to that of one or other of 

 the varieties which present themselves in the bones of the Vertebrate 

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

 the same fish, as is the case with the Lepidosteus (Fig. 442), one of the 

 few existing representatives of this Order, which, in former ages of the 

 Earth's history, comprehended a large number of important families. 

 Their name (from yavos, splendor) is bestowed on account of the smooth- 

 ness, hardness, and high polish of the outer surface of the scales; which 

 is due to the presence of a peculiar layer that has been likened (though 

 erroneously) 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 over- 

 lapping 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 allies, are 

 irregular in their shape, and very commonly do not come into mutual 

 contact, but are separately imbedded in the skin, projecting from its sur- 

 face under various forms. In the Eays 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 resem- 

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

 which extend from their centre to their circumference in minute ramifi- 

 cations, without any trace of osseous lacunae. These tooth-like scales are 

 often so small as to be invisible 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 polarized 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, etc., the method must be followed which has been already de- 

 tailed under the head of Bone ( 654). 



660. The scales of Reptiles, the feathers of Birds, and the hairs, hoofs, 

 nails, claivs, and horns (when not bony) of Mammals, are all epidermic 

 appendages; that is, they are produced upon the surface, not within the 

 substance, of the true Skin, and are allied in structure to the Epidermis 

 ( 671); being essentially composed of aggregations of cells filled with 

 horny matter, and frequently much altered in form. This structure may 

 generally be made-out in horns, nails, etc.. with little difficulty; by treating 

 thin sections of them with a dilute solution of soda; which after a short time 

 causes the cells that had been flattened into scales, to resume their globu- 

 lar form. The most interesting modifications of this structure are pre- 

 sented to us in Hairs and in Feathers; which forms of clothing are very 

 similar to each other in their essential nature, and are developed in the 



