PALiEONTOLOGICAL RECORD. 383 



therefore, abound as fossils ; but an extinct group (tbe Mosasauroids) 

 was marine in its habits, and lias consequently been pretty fvilly pre- 

 served. 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 the order seem to have frequented the sea itself. Of the 

 extinct orders of Reptiles the great Ichthyosauri, and the Flesiosauri 

 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 pre- 

 served than bii'ds, if as good ; yet their remains occur by no means 

 very rarely in certain formations. The terrestrial Deinosaurs, 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 accordance 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 the bones were filled with marrow instead 

 of air. ISText after these come the remains of birds which frequent 

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

 situations. 



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 

 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 amongst 

 the later rocks, the remains of Mammals may be foimd in tolerable 

 plenty. The strictly aquatic Mammals — "Whales, Dolphins, and the 

 like — are, of course, much more likely to have been preserved as 

 fossils than the 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 the com- 



