60 INTRODUCTION. 



be afforded by their teeth ; but these are wanting in the 

 Lancelet, and are very small in the Lampreys : so that we 

 need not wonder that these fishes are unknown as fossils. 

 The higher groups of the fishes, however, taking everything 

 into consideration, may be said to be abundantly represented 

 in a fossil condition by their scales, bones, teeth, and defen- 

 sive spines. 



The Amphibians are tolerably well represented by their 

 bones and teeth, and, as regards one extinct order, by 

 integumentary plates as well. They have also left many 

 traces of their existence in the form of footprints. Most 

 living Amphibians, however, frequent fresh waters, or spend 

 a great part of their time upon the land ; and hence 

 their remains would not be apt to be preserved in marine 

 deposits. 



The abundance of Eeptiles as fossils naturally varies much, 

 according to the habits of the different orders. Of the living 

 orders, the Chelonians (Tortoises and Turtles) are by no 

 means rare ; since many of them are habitual denizens of 

 fresh water or of the sea, whilst all are provided with a hard 

 integumentary skeleton. The Snakes are mainly represented 

 by forms which frequented water, and especially by marine 

 forms. The Lizards (Lacertilia) live mainly upon the land, 

 and do not therefore abound as fossils ; but some extinct 

 forms (the Mosasauroids) were marine in their habits, and 

 have consequently been pretty fully preserved. The 'Croco- 

 dilia, again, are so essentially aquatic in their habits, that 

 their comparative frequency in aqueous deposits is no matter 

 of wonder, especially if we recollect that many of the extinct 

 members of this order seem to have frequented the sea itself. 

 Of the extinct orders of Eeptiles, the great Ichthyosauri and 

 the Plesiosauri and their allies were marine in their habits, 

 and their remains occur in what may fairly be called pro- 

 fusion. The Flying Eeptiles, or Pterodactyles, would not 

 seem to have any better chance of being preserved than 

 Birds, if as good, yet their remains occur by no means very 

 \rarely in certain formations. The terrestrial Deinosaurs and 

 Dicynodonts, again, come very much under the laws which 

 regulate the preservation of Mammals as fossils ; and their 



