GANOID AND PLACOID SCALES. 703 



Order, which, in fonner ages of the Earth's history, comprehended 

 a large number of important families. Their name (from yavog, 

 splendour) is bestowed on account of the smoothness, 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 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 armour 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 contact ; but are separately imbedded in 

 the skin, projecting from its surface under various forms. In the 

 Rays eaeh 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 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 cartila- 

 ginous internal skeletons having entirely decayed away. — In making 

 sections of bony Scales, Spiny-rays, &c, the method must be fol- 

 lowed which has been already detailed under the head of Bone 

 (§544).* 



550. 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 (§ 561) ; being essentially composed 



* The structure of the Scales of Fishes has been most elaborately- 

 described by Prof. "Williamson in his Memoirs ' On the Microscopic 

 Structure of the Scales and Dermal Teeth of seme Ganoid and Placoid 

 Fish ' in " Philos. Transact.," 1849, and ' Investigations into the Structure 

 and Development of the Scales and Bones of Fishes' in "Philos. 

 Transact.," 1851. 



