382 THE IMPERFECTION OP TSfi" 



calcareous polypidoms of the Polyzoa, the shells of the Brachiopods^ 

 the true Bivalves and most of the Gasteropods, the hyaline shells of 

 the Pteropods, the internal supports of the cuttle-fishes, and the 

 chambered shells of the Tetrabranchiate Cephalopods, all occur com- 

 monly as fossils. The entire class of the Tunicaries, however, with 

 one or two exceptions, presents no hard structures, and is hence 

 unknown to the palaeontologist. Amongst the Gasteropoda, again, 

 the sea-slugs and their allies (J^iidibranchiata) possess no shell and do 

 not occur as fossils ; whilst the shell of the land-slugs is extremely 

 minute and is hardly known to have been preserved in sedimentary 

 deposits. Lastly, the air-breathing Molkiscs, from their habits, rarely 

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

 •are less largely represented than the marine forms, owing to the pre- 

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



_/! Vertehrata : — The majority of Vertebrate animals possess a bony 

 skeleton, so that their preservation as fossils-^so far as this is con- 

 cerned — is attended with no difficulty. Some of the Fishes, however, 

 (such as the Lancelet, the Lampreys, and the Hag-fishes) have no 

 scales, and either possess no skeleton or have one which is almost 

 wholly cartilaginous. The only evidence, therefore, which could be 

 obtained of the past existence of such fishes, would be afi"orded 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 Fishes, however, taking 

 everything into consideration, may be said to be abundantly repre- 

 sented in a fossil condition by their scales, bones, teeth, and defensive 

 spines. 



The Amphibians are tolerably well represented by their bones and 

 teeth, as well as by integumentary plates, and more especially by foot- 

 prints. Most living Amphibians, however, spend their time mainly 

 upon the land, or frequent fresh waters ; and hence their remains 

 would not be apt to be preserved in marine deposits. 



The abundance of Reptiles as fossils naturally varies much, accord- 

 ing to the habits of the different orders. Of the living orders, the 

 Chelonians (Turtles and Tortoises) are by no means rare, since many 

 of them are habitual denizens of the sea or of fresh waters, while 

 'they are provided with a hard integumentary skeleton. The snakes 

 (Ophidia) are chiefly represented by marine forms, which frequented 

 water. The Lizards (Lacertilia) live mainly upon land, and do not, 



