232 AVKS. 



connection with these characters determines the sharp definition 

 as well as the relatively great uniformity of the class, which, 

 indeed, is descended from the Saurians, but exists at the present 

 day without any forms transitional to other groups. On the 

 other hand, the remains of a group of Saurians (Archceopteryx 

 lithographica) have been discovered in the Sohlenhofen lithographic 

 siate which combine characters of the Pterodactyls with those of 

 the Birds. 



The entire structure of the body of Birds corresponds with the two 

 principal modes of locomotion on the one hand flight, and on th3 

 other walking and hopping on the earth. The trunk, which is oval, 

 is supported in an obliquely horizontal position on the two hind legs, 

 the pedal surface of which stretches over a relatively large area. 

 Posteriorly the body is prolonged into a short rudimentary tail, the 

 last vertebra of which serves for the support of a group of stiff 

 steering, or tail feathers (rectrices). In front it is prolonged into a 

 movable neck, on which is balanced a light, rounded head, with a 

 projecting, horny beak. The anterior extremities, which are trans- 

 formed into wings, lie folded together at the sides of the body. 



Arrangements for lessening the weight of the body are discernible 

 in the special structure of all the systems of organs; these are 

 .especially noticeable in the structure of the osseous skeleton. The 

 bones contain air-spaces (jpneumaticity], which communicate with 

 the air-sacs of the body through openings in the dense and firm 

 osseous substance, which is however confined to a relatively thin 

 layer. This pneumaticity is most highly developed in those birds 

 which combine a quick and enduring power of flight, with a consider- 

 able size of body (Albatross, Hornbill, Pelican). In these cases all 

 the bones except the quadrato-jugal and the scapula appear to be 

 pneumatic, while on the other hand in the Ratitce (Ostrich), which 

 have lost the power of flight, all the bones except some of the 

 cranial bones are filled with marrow. 



The Skeleton. Except in the Ostrich-like birds, the cranial bones 

 very early fuse together to form a light and firm skull, which articu- 

 lates with the atlas by means of a single condyle. The squamosal 

 and periotic bones (prootic, epiotic and opisthotic) fuse to form a 

 single bone which is united with the occipital and with which the 

 quadrate articulates. The large frontal bones take the principal 

 part in the formation of the cranial roof. Almost the whole of the 

 upper edge of the large orbit, which in the Parrots is closed by a 

 lower ring, is formed by the frontal bones. An independent lachry- 



