350 



ZOOLOGY 



SECT. 



pelvis the ilium, which usually does not articulate with the spinal column, 

 is a rod-shaped bone : the ischium and pubis resemble those of the Lizards. 

 The bones of both fore- and hind-limbs are short ; there are five digits in each. 

 The teeth are conical, pointed, and anchylosed by expanded bases to the 



1 



Fie. 052. Edestosaurus (Pythonomorpha). Pectoral arch and fore-limbs. <. coracpid ; 

 It. humerus ; me. metacarpus; r. radius; sc. scapula; ((.ulna; /, first digit; I", fifth digit. 

 (From Zittel, after Marsh.) 



summits of the maxillae and pterygoids. Dermal scutes have been observed in 

 one genus. 



The remains of Pythonomorpha have been found only in certain beds belong- 

 ing to the Cretaceous period in Europe, North America, and New Zealand. 



CLASS VI. AVES. 



In many respects Birds are the most highly .specialised of 

 Craniata. As a class they are adapted for aerial life ; and almost 

 every part of their organisation is modified in accordance with 

 the unusual environment. The non-conducting covering of 

 feathers ; the modification of the fore-limbs as wings, of the 

 sternum and shoulder-girdle to serve as origins of the great 

 wing muscles, and of the pelvic girdle and hind-limbs to enable 

 them to support the entire weight of the body on land ; the 

 perfection of the respiratory system, producing a higher tempera- 

 ture than in any other animals ; all these peculiarities are of the 

 nature of adaptations to flight. Add to them the absence, in all 

 existing Birds, of teeth, the loss of the left aortic arch, and of the 

 right ovary and oviduct, the specialised character of the brain, the 

 poorly developed olfactory organs, and the extraordinarily large 

 and perfect eyes, and we have a series of strongly-marked charac- 

 teristics such as distinguish hardly any other class. Moreover, 

 the organisation of existing Birds is, in its essential features, 

 singularly uniform, the entire class presenting less diversity of 

 structure than many single orders of Fishes, Amphibians, and 

 Reptiles. 



