30 INTRODUCTION. 



as coal and peat. There is, however, a considerable number 

 of aquatic insects (but exclusively in fresh water), and there 

 are many insects the larvae of which inhabit water, whether 

 this be fresh or salt ; so that instances of these occurring as 

 fossils are not very infrequent. 



e. Mollusca. This sub-kingdom requires little notice, since 

 the greater number of its members possess hard structures 

 capable of being preserved in a fossil condition. Thus, the 

 horny or calcareous polypidoms of many of the Polyzoa, the 

 shells of the Brachiopods, the true Bivalves, and most of the 

 Gasteropoda, the internal skeletons of the Cuttle-fishes, and 

 the chambered shells of the Tetrabranchiate Cephalopods, all 

 occur more or less abundantly as fossils. The entire class of 

 the Tunicaries, however, presents (with one or two exceptions) 

 no hard structures, and is hence not with certainty known by 

 any fossil representative. Amongst the Gasteropoda, again, 



. the Sea-slugs and their allies (Niidibranchiata) possess no shell, 

 and are unknown to the palaeontologist; whilst the shell of the 

 I Land-slugs is extremely minute, and has not been certainly 

 , recognised as fossil. Lastly, the air-breathing terrestrial Mol- 

 luscs, from their habits, rarely occur as fossils ; whilst those 

 which inhabit rivers, ponds, and lakes are less largely repre- 

 sented than marine forms, owing to the preponderance of salt- 

 water deposits over those of fresh water. 



f. Vertebrata. The majority of Vetebrate animals possess a 

 bony skeleton, so that their preservation in a fossil state so 

 far as this point is concerned is attended with no difficulty. 



^ Some of the fishes, however (such as the Lancelet, the Hag- 

 fishes, and the Lampreys), have no scales, and either possess no 

 " endoskeleton " or have one which is almost wholly cartila- 

 ginous. The only evidence, therefore, which could be obtained 

 of the past existence of such fishes would 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 defensive spines. 



The Amphibians are tolerably well represented by their 

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

 mentary plates as well. They have also left many traces of 

 their existence in the form of footprints. Most living Amphi- 

 bians, 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. 



