382 ZOOLOGY SECT. 



curved cochlea. The kidney is three-lobed, and is developed from 

 the metanephros, the mesonephros undergoing atrophy. There is 

 no urinary bladder. The ovary and oviduct of the right side are 

 more or less completely atrophied. 



Birds .are all oviparous, and the large ovum, containing much 

 food-yolk, becomes invested with albumen, a shell-membrane, and a 

 calcareous shell in its passage down the oviduct. The embryo has 

 an amnion, an allantois, and a large yolk-sac. The newly-hatched 

 young may be either well covered with down and able to run or 

 swim and to obtain their own food, in which case they are said to be 

 precocious, or may be more or less naked and dependent for a time 

 upon the parents for their food supply, when they are non- 

 precocicus. 



There is no general agreement with regard to the classification 

 of Birds. Owing to the singular uniformity of the class in essential 

 matters of structure, the vast and bewildering diversity in detail, 

 and the puzzling cross-relationships between group and group, the 

 splitting up of the class into orders is a matter of great difficulty 

 and one upon which hardly two ornithologists are agreed. The 

 following scheme will probably answer the present purpose 

 sufficiently well. 



Sub-class I. Archaeornithes. 



Mesozoic Birds having no ploughshare bone, but a long tail of 

 many vertebrae, having the rectrices arranged in two rows, one on 

 each side of it. The carpals and metacarpals are probably free and 

 the hand has three clawed digits. Teeth are present in both jaws. 



Including the single genus and species Archceoptoryx iitlio- 

 graphica, known only from two fossil specimens found in the Upper 

 Jurassic rocks of Bavaria. 



Sub-class II. Neornithes. 



Birds in which the greatly shortened tail usually ends in a 

 pygostyle, around which the rectrices, when present, are arranged 

 in a semicircle. Except in a few extinct forms there are no teeth. 

 The metacarpals are fused with the distal carpals to form a carpo- 

 metacarpus. Except in one instance not more than two digits 

 of the hand bear claws. 



Division A. Ratitae, 



Flightless Neornithes, usually of large size, having no hooked 

 barbules to the feathers, so that the barbs are free. Apteria are 

 usually absent in the adult. The rectrices are absent or irregularly 

 arranged and the pygostyle is small or undeveloped. The sternal 



