IMPERFECTION OF PAL^EONTOLOGICAL RECORD. 31 



The abundance of Reptiles 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 

 (Laccrtiliat) 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 Crocodilia, 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 fre- 

 quented the sea itself. Of the extinct orders of Reptiles, the 

 great Ichthyosauri and the Plesiosauri and their allies were 

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

 fairly be called profusion. The Flying Reptiles, or Ptero- 

 dactyles, 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 remains are chiefly, but not exclusively, to be found 

 in fluviatile 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 ac- 

 cordance with these considerations the most abundant remains 

 of Birds are referable to large wingless forms, to which the 

 power of saving themselves from their enemies by flight was 

 denied, whilst most of their bones were filled with marrow in- 

 stead of air. Next in abundance after these come the remains 

 of birds which frequent the sea-shore, lakes, estuaries, or rivers, 

 or which delight in marshy situations. 



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 



