THE SHAPES AND SIZES OF ANIMALS 153 



of the largest groups being now extinct. Of these 

 the Plesiosauri and Ichthyosauri attained lengths up 

 to twenty feet, while the flying Pterodactyls were 

 also of large size, some measuring as much as 

 twenty feet in expanse of wing ; but these 

 dimensions were greatly exceeded by other forms. 

 It has been estimated that Mosasaurus was as much 

 as seventy feet in length, while in the genus Bron- 

 tosaurus, Professor Marsh believes we have a truly 

 terrestrial reptile eighty feet in length, thirty feet 

 high, and estimated to have weighed not less than 

 twenty tons. Of recent reptiles the Crocodiles and 

 some of the Chelonians attain large dimensions, 

 while snakes are described up to thirty feet in 

 length and as thick as a man's body. 



It is however amongst Mammals that we find 

 as already noticed the real giants, and it is in- 

 teresting to note, that in different geological periods 

 different groups of Mammals have worked up to a 

 maximum of size and then disappeared. Thus the 

 Edentates at one time gave rise to giant forms 

 culminating in the huge Glyptodon and still larger 

 Megatherium, but the existing Edentates like ex- 

 isting Amphibians are a very puny race compared 

 to their forefathers. Ungulates seem in past times 

 to have had a greater number of large forms than 

 at present, though even now they are among 

 terrestrial animals second only to the Elephants. 

 It is interesting also to note that there seems to 

 be an actual limit to the size of a terrestrial 

 mammal, which has been approached more than 

 once by separate groups. It can hardly be an 



