306 VERTEBRATA. 



ment provided at some time or other with epidermic append- 

 ages in the form of hairs. The young nourished for a shorter 

 or longer time by means of a special fluid the milk secreted 

 by special glands the mammary glands. Animal viviparous. 

 As regards the general distribution in time of the Vertebrata, 

 the earliest known traces of the existence of this sub-kingdom 

 are found in the Upper Silurian Rocks. Here are the remains of 

 Ganoid and Plagiostomous fishes ; and we may fairly anticipate 

 that further research will ultimately result in putting back the 

 first appearance of Fishes at any rate to the Lower Silurian. 

 The class of the Amphibians is not known to have come into 

 existence prior to the commencement of the Carboniferous 

 period, but it had attained a great development before the 

 close of this epoch. The class of the true Reptiles is repre- 

 sented, by more or less doubtful examples only, in the newer 

 Palaeozoic deposits. In the Mesozoic Rocks, however, the 

 development of this class was so great that the Secondary 

 period has been termed the " Age of Reptiles." The class 

 Aves is doubtfully represented by foot-prints in strata of the 

 age of the Trias ; but no Palaeozoic remains of this class have 

 been as yet detected. The earliest undoubted remains of 

 Birds occur in the Jurassic series, and the class has continued 

 to be represented more or less abundantly to the present day. 

 Lastly, the class of the Mammalia, so far as at present known, 

 finds its earliest fossil representative in strata of the age of the 

 Trias (New Red Sandstone). The Mammals, however, can- 

 not be said to be in any way abundant as fossils, till we reach 

 the Eocene Tertiary. From this point onward the remains of 

 Mammals are as abundant as, in the nature of the case, they 

 could reasonably be expected to be. 



CHAPTER XXVIII. 

 FISHES. 



THE first class of the Vertebrata is that of the Fishes (Pisces), 

 which may be broadly defined as including Vertebrate animals 

 which are provided with gills throughout the whole of life ; the 

 heart, when present, consists (with one exception} of a single auricle 

 and a single ventricle ; the blood is cold ; the limbs, when present, 

 are in the form of fins, or expansions of the integument. 



In form, Fishes are adapted for rapid locomotion in water, 

 the shape of the body being such as to give rise to the least 



