IMPERFECTION OF PALjEONTOLOGICAL RECORD. 6/ 



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

 ding 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 well-developed skeleton. The exist- 

 ing Squamata (Lizards and Snakes) and the Rhynchocephalia live 

 chiefly upon the land, and do not therefore abound as fossils ; but 

 some extinct types of the former (the Mosasauroids) were marine in 

 their habits, and have consequently been pretty fully preserved. 

 The Crocodi/ia, 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 frequented the sea itself. Of the extinct orders of Rep- 

 tiles, the great Ichthyosaurs and the Plesiosaurs and their allies were 

 marine in their habits, and their remains occur in what may fairly 

 be called profusion. The Flying Reptiles, 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 Dinosaurs and Anomodonts, again, come 

 very much under the laws which regulate the preservation of Mam- 

 mals as fossils ; and their remains are chiefly, but not exclusively, 

 to be found in fluviatile or estuarine deposits. 



As regards Birds, their powers of flight, as pointed out by Sir 

 Charles Lyell, would save them from many destructive agencies, and 

 the lightness of their bones would favour the long floating of the 

 body in water, and thus increase the chances of its being devoured 

 by predaceous animals. In accordance with these considerations, 

 the most abundant remains of Birds belong to species which fre- 

 quent the sea-shore, lakes, estuaries, or rivers, or which delight in 

 marshy situations ; though in certain regions the principal fossil rep- 

 resentatives of the class Aves are large wingless forms, of terrestrial 

 habit, and with their bones largely filled with marrow instead of air. 



Lastly, as regards Mammals, the record is far from being a full 

 one, and from obvious causes. The great majority of Mammals 

 live on land, and therefore are not likely to be buried in aqueous, 

 and especially in marine, accumulations. That this cause is the 

 chief one which has operated against the frequent preservation of 

 Mammalian remains is shown by the fact that when we exhume an 

 old land-surface, the remains of Mammals may be found in tolerable 

 plenty. The strictly aquatic Mammals — such as Whales, Dolphins, 

 and the like — are, of course, much more likely to have been pre- 

 served as fossils than the strictly terrestrial forms ; but their want of 

 integumentary hard structures places them at a disadvantage in this 

 respect as compared with fishes. In a general way, we may conclude 

 that the preservation of the terrestrial Mammals as fossils is due to- 



