xni PHYLUM CHORDATA 397 



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-precocious. 



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. Archaeornithe-s. 



Mesozoic Birds : have no ploughshare bone (pygostyle), 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 Archceoptoryx with three species, 

 known only from three specimens found in the 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, and in nearly all cases claws are absent in 

 the manus. 



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 

 keel is vestigial or absent. The coracoid and scapula are com- 

 paratively small and completely ankylosed ; the acrocoracoid process 

 is vestigial, and the coraco-scapular angle approaches two right 

 angles. The wing is reduced in size and may be vestigial or 

 absent. There are large basi-pterygoid processes developed from 

 the basi-sphenoid. The vomer is large and broad. The quadrate 



