290 



VERTEBRATE SKELETON 



corresponding digit. Most birds have four digits in the pes, the 

 tibial one turned backwards, and in parrots (fig. 314) and many other 

 cHmbing birds the last toe on the fibular side is also reversed. In 

 some 'Palmipeds' all four toes are directed forwards. Casso- 

 waries, Rhea and some woodpeckers have only three digits, the 

 African ostrich {Struthio) but two, apparently 3 and 4. There is 



Httle evidence as to whether digit i 

 or 5 is lost in most birds; the usual 

 statement is that i, 2, 3, and 4 

 ^(Jf"^/^ persist. 



Arch(Bopteryx has a relatively strong 

 femur, distinct tarsal bones and four 

 toes. The fibula is free in moas, longest 

 in penguins (which have short metatar- 

 sals) and some Raptores where it nearly 

 reaches the tarsal joint. The 'Dorking 

 fowl ' are the only exception to the rule 

 of four toes or less, these having five, 

 the additional toe being thought to arise 

 by the division of i, but explicable also 

 on the supposition that the first, not 

 the fifth is lost in other birds. This 

 fowl also has an increase in the number 

 of phalanges. 



MAMMALIA. — The mam- 

 mahan appendages, primitively 

 adapted for walking and running, 

 undergo, in the several orders 

 greater modifications, especially in the fore hmb, than in other classes 

 of Vertebrates, becoming changed into organs for leaping, digging, 

 grasping and climbing, swimming or flying; uses paralleled to some 

 extent in Sauropsida. Yet, in all mammals, the Tetrapod structure 

 is retained. The more primitive groups are plantigrade, this feature 

 being retained even in Primates. Others are digitigrade, and lastly 

 Ungulates and Proboscidia are unguhgrade. 



In Hmb structure the lower mammals are primitive, and much hke 

 the Stegocephals and the less differentiated Theromorphs; in the 

 more specialized mammals several modifications appear. The fore 

 Hmb has become strongly reversed, the elbow pointing backwards, 

 resulting in a twisting of the humerus so that the dorsal side of the 



Fig. 314.- — Foot of parrot (P. amazoni- 

 cus). /, femur; fb, fibula; p, patella; tm, 

 tarsometatarsus; tt, tibio-tarsus; II-V, 

 digits. 



