108 BULLETIN UNITED S'l'ATES GEOLOGICAL SURVEY. [Yol.YI. 



as in Aceipiter coopet'i; in very many birds it is absent. The scapula pre- 

 sents little that is unusual in that bone among the class generally. It 

 lends the additional two-thirds of articular surface to form the glenoid 

 cavity with the coracoid; internal to this the acromion i)rocess extends 

 forwards, touching the coracoid as described, and having a limited bear- 

 ing on the clavicle. Posteriorly its blade-like length, is produced, ex- 

 panding, turning slightly outwards to terminate in an obliquely trun- 

 cate extremity, with its point over the second dorso-pleurapophysial 

 interspace. 



What the scapula lacks in interest is amply made up by the changes 

 observed in the last bone of the group, the clavicle. This element is 

 broad above, much comi)ressed from side to side throughout ; it spans 

 the anterior groove of the coracoid and touches the scapula as described 

 above, rapidly diminishing in size as it is produced downwards and in- 

 wards by a gentle curve towards the fellow of the opposite side. The 

 upper extremities in adult birds are separated by an average distance 

 of 2.3 centimetres. If the sternum pointed to feebleness of flight in 

 this little Owl, it is still further carried out by the ill-developed clavicles, 

 which constitute that arch in birds, where they are thoroughly and 

 firmly united below, that assists to resist the pressure of the humeri when 

 the wings are depressed in flight, and send them back to their former 

 l^osition after the completion of the action. In examining again PI. I, 

 which represents the skeleton of an old male, we find this bone to be 

 simply a pointed styliforra process ; in other individuals, and adults too, 

 it does not even attain the length here shown ; but, as if to bid defiance 

 to all law or invariable rule governing it, we again find in very young 

 birds cases where it becomes confluent with its fellow, forming a broad 

 U-shaped arch, though never a very strong one. In a case of this kind 

 the bone was finely cancellous throughout, with an extremely attenuated 

 layer, scarcely covering it outside, of compact tissue. In PI. I, and 

 other individuals like it, the clavicles were pneumatic. Again, in both 

 young and old, it may have any of its lower i)arts completed by carti- 

 lage, or unite with tlie manubrium by means of the same material ; it 

 never displays a mesial expansion of bone at the point of confluence. 

 As already shown, the superior entrance of the anterior groove on the 

 coracoid is a complete circuit, formed by the three bones of the group. 

 The head of the coracoid overhangs it above ; next below is the clav- 

 icle, closing it in anteriorly ; lowest of all the scapula behind. A plane 

 l^assed through the superior margins of this aperture would look up- 

 wards, iuwards, and backwards. All the bones of the scaiJular arch are 

 pneumatic, with the exception sometimes seen in the clavicle, and the 

 foramina, to allow the air to enter their interiors, look into the enclosed 

 groove of the coracoid just described. In the scapula the foramen is 

 usually single and in the acromion process, single again in the clavicle j 

 it is seen in the broadest part of the head, while in the coracoid there 

 is generally a group of these little apertures, situated in the depression 

 on the surface that overhangs this entrance to the coracoidal groove. 



