LIFE IN PRIMARY PERIOD 229 



transformative action and have come down to us unchanged, 

 whereas others have been obedient to it and have become 

 modified. This action, moreover, has been exercised equally 

 on the trunk, properly so called ; gradually the purely 

 muscular and impulsive part of the body constituting the tail 

 increased at its expense, whilst at the same time the currents 

 it produced forced forward the pelvic fins, which were 

 primitively situated at a distance from the pectoral, until 

 they were placed underneath and in front of the pectoral fins 

 and articulated with the branchial skeleton itself. This, 

 as we have seen, is the characteristic of the swimming 

 Fishes par excellence, the fishes of the open sea. 



The Marsipobranchs must therefore be considered the 

 oldest of all Fishes. They show no tendency to secrete lime 

 and their skin is absolutely denuded of any solid product. 

 However, in the buccal cavity, where it is naturally subjected 

 to incessant friction, the epidermis of its papillae, in Lampreys, 

 takes on a horny consistency, and produces short pointed spines, 

 broad-based and conical in shape, which play the part of teeth, 

 but which are only the antecedents of teeth and are known 

 as odontoids. The epidermis of the Elasmobranchs, on the 

 contrary, becomes calcified all over, but in such a way that 

 it becomes a sort of a mosaic of small thick scales, circumscribed 

 by linear intervals where the epidermis remains flexible. 

 This epidermal structure extends to the mouth as well, and 

 is identical with that of the dental enamel of all the other 

 vertebrates. In certain parts this calcification continues 

 below the epidermis to the superficial portion of the dermis, 

 to which the living cells that have produced it send fine 

 prolongations without themselves penetrating it. Solid plates 

 are thus formed, more or less covered by enamel, and sometimes 

 with sockets carrying spurs, as, for example, in the armature 

 of the Rays. These plates and sockets, formed of calcareous 

 incrustations and traversed by fine vessels, have the same 

 structure as dental ivory, and, as they are covered with enamel, 

 the teeth of the terrestrial vertebrates must be regarded 

 as the last remnants of the defensive armour of the Elasmo- 

 branch Fishes, finally localized on the jaw. 



The Elasmobranchs have no bones. Their vertebrae, it is 

 true, may become calcified, but this calcification, which 

 takes place in various ways, does not modify their internal 



