CLASSIFICATION OF BIRDS. 617 



. to realise that, as a result of this, the still small embryo is attached by a 

 relatively narrow stalk to the large yolk sac, over which the blastoderm 

 is now slowly spreading. In the young tadpole, the yolk lies heaped up 

 on the floor of the gut, and causes a certain amount of distortion. In 

 the chick the amount of yolk is so great that it forms a hernia-like pro- 

 trusion of the gut, and only at a very late stage is the greatly reduced 

 sac withdrawn into the gut, after which the dermal and intestinal 

 umbilical openings are closed. 



With regard to the development of the various organs of the body, the 

 conditions are much the same as for the frog. The chick embryo never 

 exhibits any trace of gills, but yet gill clefts perforate the pharynx. The 

 embryonic organ of respiration is the allantois, but yet that arrangement 

 of aortic arches by means of which in the tadpole blood is carried to the 

 gills, is repeated here. 



About the twentieth day, the beak perforates the membranes of the 

 air chamber, and, the air rushing in, expands the hitherto functionless 

 lungs. At the same time important changes occur in the circulatory 

 system, " the umbilicus becomes completely closed, the allantois shrivels 

 up, and the chick, piercing the broad end of the shell with repeated 

 blows of its beak, steps out into the world." 



CLASSIFICATION OF BIRDS. 



I. Sub-Class. ARCH^EORNITHES or SAURUR^:. Ancient extinct birds, 

 with reptilian affinities more marked than in any living forms. 



The oldest known bird is Archczopteryx, remains of which have been 

 found in the Solenhofen slates in the Upper Oolite (Jurassic) of Bavaria. 

 "The stone is so fine grained that, besides the bones of the wings, the 

 furculum or merrythought, the pelvis, the legs, and the tail, we have 

 actually casts or impressions on the stone (made when it was as yet only 

 soft mud) of all the feathers of the wings, and of the tail." (Nicholson 

 and Lydekker). 



This link between Birds and Reptiles seems to have been a land bird 

 about the size of a crow. It had feathers, and the upper jaw bore 

 conical teeth. In the fossil specimen the feathers are confined to the 

 wings, legs, and tail, those on the head, neck, and trunk, having 

 perhaps fallen off. Each joint of the long tail bears a pair of lateral 

 feathers a unique arrangement. The vertebrae have flat ends ; the 

 ribs are very slender, without uncinate processes ; the sternum is not 

 known ; there seem to have been abdominal ribs. There are separate 

 metacarpals ; the first finger has two phalanges, the second three, the 

 third three or four, and all are clawed. Dr. Hurst has recently main- 

 tained that the metacarpals of digits 4 and 5 are also present. The tail 

 was long like that of a lizard, with about a score of vertebrae, and no 

 ploughshare bone. 



II. Sub-Class. NEORNITHES. 



The metacarpals are fused. The second finger is the longest, and the 

 third is reduced. Caudal vertebrae are apparently not more than thirteen 

 in number. 



