XIII PHYLUM CHORDATA 409 



species the distal carpals and the metacarpals are united to form a 

 carpo-metacarpus. There are usually oaly three digits in the wing, 

 which probably represent the first, second, and third of the typical 

 hand. The ilium is of great size, having large pre- and post- 

 acetabular portions. The acetabulum is perforated in the dry 

 bone. The pubis and ischium are directed backwards, and, except 

 in one case of each, there is neither pubic nor ischiadic symphysis. 

 The head of the femur is at right angles to the shaft. The 

 proximal tarsals are fused with the tibia to form a tibio-tarsus ; 

 the fibula is much reduced. The distal tarsals are fused with the 

 second, third, and fourth metatarsals to form a tarso-metatarsus ; 

 the first metatarsal is free. The fifth digit of the typical foot is 

 absent. 



In all tertiary and recent Birds teeth are absent. The gullet is 

 frequently dilated into a crop, and the stomach is usually divided 

 into proventriculus and gizzard. The junction between the large 

 and small intestines is marked b}' a pair of cseca. The luDgs are 

 spongy and non-distensible. The bronchi give off branches which 

 open on the surface of the lung into thin-walled air-sacs, and these 

 in their turn usually communicate with pneumatic cavities in more 

 or fewer of the bones. The voice is produced in a syrinx situated 

 at or near the junction of the trachea with the bronchi. The heart 

 is four-chambered, the right auriculo-ventricular valve is muscular, 

 and the right aortic arch alone is present in the adult. The renal 

 portal system is vestigial. The red blood-corpuscles are oval and 

 nucleated. The temperature of the blood is high (about 38° C). 

 The optic lobes are displaced laterally owing to the meeting of the 

 large cerebral hemispheres and cerebellum. The lumbar region of 

 the spinal cord has a sinus rhomboidalis. The olfactory organ is 

 usually poorly developed. The eye is usually large, and has 

 sclerotic plates and a pecten. The auditory organ has a large, 

 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 double shell-mem- 

 brane, 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 he 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 



