378 ZOOLOGY sect. 



venti'ally, and a, scapula '(.w.) which resembles that of the Ehynchocephalia : 

 a clavicle is never present. In the 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 

 ankylosed by expanded bases to the 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 v.— 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 enviroment. 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 the surface of the 

 ground ; the perfection of the respiratory system, producing a 

 higher temperature than in any other animals : all these peculiari- 

 ties 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 extra- 

 ordinarily large and perfect eyes, and we have a series of strongly- 

 marked characteristics 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. 



1. Example of the Class. — The Common Pigeon (Columha 

 livia, var. domestica). 



The Common or Domestic Pigeon is known under many varieties, 

 which differ from one another in size, proportions, coloration, 

 details in the arrangements of the feathers, and in many points of 

 internal anatomy. The Pouters, Carriers, Fantails, and Tumblers 

 may be mentioned as illustrating extreme forms. All these 

 varieties have, however, been produced by artificial selection, 

 that is, by breeders selecting, generation after generation, the 

 Birds which most nearly attained to some artificial standard of 

 perfection, breeding from them alone, and killing off the inferior 

 strains. The ancestral species from which the domestic breeds 

 have in this way been evolved, is the Bock Pigeon (^Columba livia), 



