^86 MAMMALS. PHYLUM CHORDATA 



be primarily an animal of the fir woods, which once covered most 

 of Britain but now persist, otherwise than as plantations, only in 

 some restricted parts of the Highlands of Scotland. It is not there- 

 fore surprising that the red squirrel is not a very successful species 

 under present conditions. It has, however, increased in some 

 districts in recent years. Squirrels are among the more primi- 



X o 2 3 



tive rodents, with dental formula 



and a more normal 



I0I3 



arrangement of the jaw musculature than the mice. The grey 



squirrel (5. carolinensis) was introduced from 

 North America at various places from 1876 

 to 1910, and is now common in woods and 

 parks over most of midland and southern 

 England. 



Several other species of rodent are known as 

 fossils ; the most remarkable is the beaver 

 [Castor fiber), a large aquatic species notorious 

 for its habit of building dams with carefully 

 fitted logs of wood. It persisted in Wales into 

 historic times. 



CETACEA 



Fig. 376. — The left 

 fore-limb of Balcs- 

 noptera, a whale- 

 bone whale. — 

 F"rom Thomson. 



Sc, Scapula with spine 

 (Sp.) ; //., humerus ; 

 /?., radius ; U., ulna ; 

 C, carpals embedded 

 in matrix ; Mc, meta- 

 carpals; PA., phalanges. 



The third cohort, Mutica, contains only one 

 Order, the Cetacea, or whales and dolphins, 

 which, except that they still breathe air, have 

 become completely aquatic. In so doing they 

 have come to have considerable superficial 

 similarity to fishes. They have the same stream- 

 lined shape, they swim by a caudal fin, and 

 they balance by the fore-limbs and unpaired 

 fins, but the flukes of the tail are horizontal instead of vertical, 

 and neither they nor the unpaired fins contain any skeleton. 

 That of the fore-limb, although reduced, is recognisable as coming 

 from the pentadactyl type (Fig. 376). The teeth are with few 

 exceptions either many, conical, peg-Uke and of a single set only, 

 or completely absent ; if they are present the animals are usually 

 piscivorous, if they are absent the animals have returned to the 

 microphagy of their early chordate ancestors (p. 304), and filter 

 the water with combs made of baleen or whalebone, which is in 

 fact keratin. Whales have a thick layer of fat or blubber beneath 



