366 



ZOOLOGY 



SECT. 



scapula (sc.) which resembles that of the Rhynchocephalia : 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 



FIG. 1032. Edestosaurus (Pythonomorpha). Pectoral arch and fore-limbs, c. coracoid with 

 pro-coracoid ; h. humerus ; me. metacarpus ; r. radius ; sc. scapula u. ulna ; 7, first 

 digit ; V, fifth digit. (From Zittel after Marsh.) 



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 

 belonging to the Cretaceous period in Europe, North America, and New 

 Zealand. 



CLASS V.-A7ES. 



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 the surface of the ground ; the perfec- 

 tion of the respiratory system, producing a higher temperature 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 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. 



