308 PALEONTOLOGY. 



moveable on each other. The shoulder arch is formed of 

 two clavicles united together at their lower ends, to form 

 a Y-shaped bone, known as the merry-thought, two long 

 blade-shaped scapulae, and two strong coracoids. These six 

 bones are firmly tied together with ligaments. The coracoids 

 are articulated to the sternum, and resist the too near 

 approximation of the arm bones to the median line in the 

 down stroke which the wing makes in the act of flying. 

 The sternum is a broad plate of bone which covers the 

 greater part of the inferior surface of the body, as in 

 Chelonia. It has, in general, a median elevated crest, destined 

 to give a more extensive attachment to the pectoral muscles. 

 The pelvis is formed of three bones on each side, as in 

 reptiles and mammals, but in birds they are soldered together 

 into a single piece, which extends forwards and backwards 

 from the cavity of the hip-joint along the sides of the sacrum 

 as in saurian reptiles. 



In the anterior extremities we find an arm-bone; two 

 bones in the fore-arm, and carpal, meta-carpal and phalanges! 

 bones. In the posterior extremities is a thigh bone, two leg 

 bones, a tarsus, and a meta-tarsus, formed of a single bone, 

 which presents three pulley-like surfaces on its lower end, for 

 the attachment of the three toes which are directed forwards, 

 and one for the small toe or thumb which is directed back- 

 wards. The number of joints increases in each toe, com- 

 mencing with the thumb, which has two, the second toe 

 three, the third toe four, and the fourth toe, which has five 

 phalanges. 



The skull is small and is composed of thin, compact bones 

 firmly united together. The orbits are large; the upper 

 jaws are greatly developed by the elongation of the bones of 

 the face, and the lower jaw is articulated to a tympanic 

 bone as in reptiles. 



The jaws are covered with a horny coating, as in Chelonia, 

 and are adapted as organs of prehension. The uniformity 

 of the typical character of the skeleton, renders the osteology 

 alone an imperfect means for determining the orders of this 

 class. The bill and the feet, so important to the zoologist, 

 are unfortunately not often preserved in a fossil state. For 

 these reasons, our knowledge of fossil birds is much behind 

 that of the other classes of the vertebrata. 



