AVES 281 



flat sternum, and by the fact that the well-developed legs were set far 

 back as in modern penguins. This species, Hesperornis (Fig. 151), 

 was a large bird about four feet in length. It had a large head and its 

 jaws were provided with true teeth imbedded in sockets of the max- 

 illary and dentary bones. In general appearance it must have re- 

 sembled the modern loons except for the wings which were very much 

 reduced, consisting merely of a long, slender humerus, without any 

 fore-arm or hand. 



Fragmentary remains of another toothed diving bird, Baptornis, 

 have also been referred to this division. 



BIRDS OF TO-DAY 



The Present Status of Birds. The birds of to-day rank with 

 the teleost fishes as a climax group. They appear to be at the height 

 of their evolution and have undergone a very elaborate adaptive 

 radiation, being specialized for life in the trees, for life on and under 

 the ground (in caves), for life in waters shallow and deep, and for 

 life in the air. They have many specialized types of diet: carnivorous, 

 insectivorous, herbivorous, and graminivorous. 



Modern birds show also many signs of racial senescence, especially 

 in their extreme specializations of beaks and of feet, in their over- 

 elaborate integumentary structures, and in their riotous coloration. 

 The various types of beaks and feet are well shown in Figs. 152 and 

 153, and explained in the legends. 



Birds exhibit very pronounced sex-dimorphism, the males usually 

 being more highly colored and with more elaborate plumage, wattles, 

 spurs and other excrescences; while the females are usually colored 

 more like the background and are in other ways much less specialized. 

 Birds differ from all other vertebrates in that the female is the 

 heterozygous sex, yielding two kinds of eggs, male-producing and 

 female-producing; whereas in other vertebrates it is the male that is 

 heterozygous and produces male and female sperms. 



THE RUNNING BIRDS (NEORNITHES RATIT^E) 



This division of modern birds is comparatively small in number of 

 species; but they make up for their small numbers by their large size. 

 They are characterized by: absence of keel to the sternum; greatly 

 reduced wings incapable of flight; coracoid and scapula fused together; 



