66 INTRODUCTION. 



a long and tolerably complete palaeontological record. In the case 

 of the Polyzoa, however, the group of the fresh-water forms is not at 

 present known to have any fossil representatives. 



g. Molhisca. — This sub-kingdom requires little notice, since the 

 greater number of its members possess hard structures readily cap- 

 able of preservation in the fossil condition. Hence, all the great 

 existing groups of Molluscs are more or less extensively represented 

 in past time. The oceanic Pteropods, however, perhaps owing to 

 their pelagic habit of life, or possibly as the result of their having 

 fragile and easily destructible shells, have left a more incomplete 

 record of their past existence than is the case with the other 

 classes of Molluscs. Amongst the Gastropoda, again, the Sea- 

 slugs and their allies (Nudibraitchiata) possess no shell, and are 

 unknown to the palaeontologist ; whilst the shell of the Land-slugs is 

 extremely minute, and has only rarely been recognised as fossil. 

 Lastly, the air-breathing terrestrial Molluscs, from their habits, rarely 

 occur as fossils ; whilst those which inhabit rivers, ponds, and lakes 

 are less largely represented than marine forms, owing to the prepon 

 derance of salt-water deposits over those of fresh water. 



h. Tunicata. — The Tunicates, so far as living forms are concerned, 

 are almost always destitute of skeletal structures, or possess only 

 microscopic spicules of carbonate of lime. Only the most limited 

 traces of the past existence of these animals have as yet been cer- 

 tainly detected ; but it is quite probable that further investigations 

 will considerably extend our knowledge on this point. 



i. Vertebrata. — The majority of Vertebrate 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 Lam- 

 preys), have no scales, and either possess no " endoskeleton " or 

 have one which is wholly or almost wholly unossified. The only 

 evidence, therefore, which could be obtained of the past existence 

 of such fishes would be afforded by their teeth ; but these are want- 

 ing in the Lancelet, and are small and horny 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 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 likely to be preserved in marine 

 deposits. 



