1028 VERTEBKATED ANIMALS 



structure, the fibres of each lamina being inclined at various angles 

 to those of the lamina above and below it. Between these two layers 

 is interposed a stratum of calcareous concretions, resembling those 

 of the scale of the eel ; these are sometimes globular or spheroidal, 

 but more commonly * lenticular,' that is, having the form of a double 

 convex lens. The scales which resemble those of the carp in having 

 a form more or less circular, and in being destitute of comb-like 

 prolongations, are called cycloid ; and such are the characters of 

 those of the salmon, herring, roach, &c. The structure of the ctenoid 

 scales (fig. 759), which we find in the sole, perch, pike, &c., does not 

 differ essentially from that of the cycloid, save as to the projection 

 of the comb- like teeth from the posterior margin ; and it does not 

 appear that the strongly marked division which Professor Agassiz 

 has attempted to establish between the ' cycloid ' and the ' ctenoid ' 

 orders of fishes, on the basis of this difference, is in harmony 

 with their general organisation. Scales of every kind may become 

 consolidated to a considerable extent by the calcification of their 

 soft substance ; but they never present any approach to the true 

 bony structure, such as is shown in the two orders to be next ad- 

 verted to. 



In the ganoid scales, on the other hand, the whole substance of 

 the scale is composed of a material 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. 

 753), 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 yaj'oc, splendour) is bestowed 

 on account of the smoothness, hardness, and high polish of the outer 

 surface of the scales, which are due to the presence of a peculiar layer 

 that has been likened to the enamel of teeth. 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 charac- 

 terise 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 

 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 

 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 polarised light. 



