DR. PETTIGREW ON THE MECHANISM OF FLIGHT. 209 



the Sea-Bear; while the posterior extremities possess many of the peculiarities of 

 hind legs of the Sea-Bear, hut display the movements peculiar to those of the 



Sc 



In other words, the anterior extremities or nippers of the Walrus are moyed alternately, 

 and reciprocate, as in the Sea-Bear: i. e. they are carried away from the body, and 

 advanced during extension, when the back or non-effective stroke is made, and drawn 

 towards the body and retracted during flexion, when the effective or forward stroke is 

 given; they are, further, rotated on their long axes, or tilted, during either movement, 

 so as to present their maximum and minimum of surface to the water alternately. Tin 

 posterior extremities, on the other hand, are lashed from side to side by a twisting 

 curvilinear motion, precisely as in the Seal. The Walrus may therefore, as far as the 

 physiology of its extremities is concerned, very properly he regarded as holding an inter- 

 mediate position between the Seals on the one hand and the Sea-Bears or Sea- Lions on 



the other. 



Swimming of the Turtle, Triton, Crocodile, &fc.— The swimming of the Turtle (Plate XII 

 fig. 7) differs in some respects from that of all the others. While the anterior extremities 

 of this quaint animal move alternately, and tilt or partially rotate during the effective and 

 non-effective strokes, as in the Sea-Bear and Walrus, the posterior extremities likewise 

 move by turns. As, moreover, the anterior extremity of one side and the posterior 

 extremity of the opposite side move together, and reciprocate with the corresponding 

 members of the other side, the creature has the appearance of walking in the water. The 



remarks apply to the movements of the extremities of the Triton and Crocodile, when 



same 

 swimm 



and to the feebly developed corresponding members in the Lepidos 



Proteus, and Axolotl, specimens of all of which are to be seen at present in the Zoolo- 

 gical Society's Gardens. In these, however, natation is effected principally, if not alto- 

 gether, by the tail and lower half of the body, which is largely developed and flattened 



laterally for this purpose, as in the fish. 



The muscular power exercised by the fishes, the Cetaceans, and the Seals, m swim- 



a remarkable extent by the momentum which the body rapidly 



the velocity attained by the mass diminishing the degree of exertion required 



the individual or integral parts. This holds true of all animals, whether they mov< 



mmg 

 acquir 



d to 



the land or on or in the water 



The animals which furnish the connecting link between the water and the air are the 



fishes (Plate XII. fi<*. 12) on the one hand and the diving birds (Plate XII. fig. 5) 



S"c*h«,-7he fome^tainmg themselves for considerable intervals in the air by 



means of their enormous pectoral fins, the latter using their wings for flying above and 

 through the water, as occasion demandst. 



• A good illustration of the enormous speed and force acquired by a fish , i. motion may be seen in the Museum 

 of the Eoval Collet of Surgeons of England, where a portion of the timbers of one „ Her Majesty . slaps „ extahted 



ha!t ^ tusk of a Sword-fish imbedded i„ it. On this occasion the formidable weapon perforated the copper 



if. i t r u i 1-npV.P* of deal and 14 inches of oaken plank, 



sheathing a laver of felt, 4 menes or utai, auu *.-* i 



t T a S eair or Race-horse Duck (Micropterus Irackypteru*) constitute, a striking link between the genera Anas 



Ill J f „ nlike because of its great swimming- and diving-powers, and because ot its short and remarkably 

 and Aptenodytes, alike because its g j th|B throu h the water . C English 



small wings, which are useless as organs ot flight, ana serve to ! j 



Cyclopedia of Natural History, vol. iii. p- 810.) 



