60 ANIMAL LIFE PAST AND PRESENT. 



the leg of the Penguins suggest, however, that these 

 birds belong to a very primitive type. 



We must not conclude our notice of swimming birds 

 without reference to the extinct Hesperornis, of the 

 Cretaceous beds of the United States. This remarkable 

 bird, which was nearly six feet in length, shows evidence 

 of its relationship to reptiles by the retention of a 

 complete series of sharp-pointed teeth in both jaws. 

 In the structure of its bones it appears to come nearest 

 to the Grebes and Divers, but it differs from all other 

 swimming birds in having lost (so far as can be deter- 

 mined) all traces of wings; and thus affords one of 

 several instances where species long extinct are in 

 certain respects more specialised than any of their 

 living relatives. 



Our remaining examples of Swimming Animals are 

 taken from the class of Mammals, or Quadrupeds, as 

 they are often popularly, though inconveniently, termed. 

 And we shall find that in certain members of this 

 group the adaptation to an aquatic life has been so 

 complete as to have led to the loss of all external 

 features characteristic of ordinary members of the 

 class, and has thus induced the erroneous popular 

 belief that the animals in question are really fishes. 



In several groups of Mammals we find that a few 

 species, or genera, have been more or less modified so 

 as to become expert swimmers and divers. Instances 

 of these are afforded by the Australian Duck-Bill 

 among the egg-laying Mammals, the Otter among the 

 Carnivores, the Beaver and Water- Vole among the 



