148 LIFE ON THE EAETH. 



kind is very faint; and a very large proportion of 

 the fossil shells of all orders must be passed over as 

 yielding no sufficient data for a sound conclusion. 

 If we may trust to the few species of the recent 

 genus Nautilus, as indicative of a warm climate, and 

 include all the fossil groups in the same inference, 

 the conclusion already obtained as to the Palaeozoic 

 and Mesozoic seas of the north temperate zone would 

 be confirmed. 



With much confidence we may appeal to the class 

 of Reptiles for proof of the warm climates of the 

 land, sea, and fresh water of the whole Mesozoic 

 period, for this whole class, as represented in modern 

 times, has such a dependence on temperature as to 

 diminish rapidly in number and dwindle to small 

 size beyond the tropics, and to require special pro- 

 visions for enduring the winter cold. Not that it 

 seems necessary to suppose, for their comfortable life, 

 a climate heated above ordinary temperatures, in the 

 same proportion that their magnitude exceeds that 

 of recent species. The living Crocodilia are con- 

 fined to rivers which open into warm seas, and their 

 own geographical range does not pass the isothermal 

 of 64 mean annual temperature. Within this range 

 are most of the races of Serpents and Batrachians on 

 land, and of Turtles in the sea; except that now 

 and then some species wander beyond their usual 



