98 COMPARATIVE ANATOMY AND. PHYSIOLOGY. 



II. Cariaiatrc, in which the ventral surface of 

 the sternum is typically provided with a median keel, 

 and the fore-limbs may serve as functional wings. 

 Among them we find the singing birds, parrots, owls, 

 eagles, geese, pigeons, and gulls ; and here, as among 

 the Teleostei, we find the most varied elaborations in 

 the details of a structural organisation, which is, in its 

 essential points, extraordinarily similar throughout 

 the group. The extinct Odontornithes (e.g. Hes- 

 perornis) were true birds with teeth in their jaws. 



The Mammalia, or last division of the Verte- 

 brata, are all distinguished from the Sauropsida by 

 the possession of two occipital condyles, and by the 

 fact that the single aortic arch is the left and not the 

 right member of a primitive pair. They are all more 

 or less hairy, and have mammary glands ; the quad- 

 rate becomes the malleus among the auditory 

 ossicles, the blood is hot, and the red blood corpuscles 



are without a nucleus, while 

 the cerebral hemispheres have 

 a corpus callosum. (See 

 page 426.) They exhibit three 

 well-marked grades of develop- 

 ment : 



A. Prototheria (Ornitho- 

 delphia\ in which the mammary 

 glands are without teats, the 

 young are not nourished within 

 the uterus of the mother by 

 means of a placenta, the 

 Fig. 48. Pelvic Arch of epipufoes (see page 348) are 



lar S 6 ' aild the coracoids 



complete. Here are placed the 

 duck-bill (Ornithorhynchus) and the Echidna (Fig. 48), 

 which have so far diverged, like the Chelonia, froni 



