XIII 



PHYLUM CHORDATA 



405 



-pb 



Fir;. 1010. Gallus bankiva (common Fowl). 

 Innominate of a six days' embryo. II. ilium ; 

 Is. ischium ; i>b. pubis ; />/i. pectineal process. 

 (From Wiedersheim, after Johnson.) 



The bones of the hind-limb are very uniform throughout the 

 class, but the form of the 

 tarso-metatarsus of Penguins 

 is worthy of notice. It is 

 short and wide, its three con- 

 stituent inetatarsals, though 

 fused, are clearly distinguish- 

 able throughout their whole 

 length, and the resemblance 

 to the homologous part in 

 Iguanodon is very striking. 

 In the embryo (Fig. 1011) a 

 vestige of the fifth digit (mt. 

 tsl. 5) has been found in the 

 form of a small rod of cartilage 

 on the postaxial or fibular side. 

 One or two free central ia may 



i c ' 



occur in the mesotarsal joint 



(Fig. 1007). 



The skeleton is always more or less pneumatic, but there is no 



definite relation between pneumaticity and power of flight. A very 



usual arrangement is for all the 

 bones to contain air except those 

 of the fore-arm and hand, shank 

 and foot. But in Apteryx, Pen- 

 guins, and some Song-birds the 

 skull alone is pneumatic, while in 

 the Hornbill every bone in the 

 body contains air. 



Myology. As might be in- 

 ferred from a study of the skele- 



t/ 



ton, the muscles of flight undergo 

 a great reduction, often amount- 

 ing to complete atrophy, in the 

 Ratitse, and to a less degree in 

 the flightless Carinatse. The pre- 

 sence or absence of an ambiens 

 and of certain other muscles in 

 the leg and in the wing furnish 

 characters of considerable classi- 

 ficatory importance. 



Digestive Organs. In all ex- 

 isting Neornithes the jaws are 

 covered by a horny beak and 

 there are no teeth. But that teeth 

 were present in the more primitive 

 Birds, and have gradually been lost during the evolution of the 



FIG. 1011. Apteryx oweni. Left hind- 

 limb of embryo, dorsal aspect, dist. 

 distale ; fc. femur ; Fib. fibula ; HI. fibu- 

 lare ; Mt. tsl. 15, metatarsals ; Tib. tibia ; 

 tib. tibiale. (After T. J. Parker.) 



