. The structure and classification of birds . atite birds has something to do with their loss of the powerof flight. In the ratite birds and in Hesperornis the scapulaand the coracoid are nearly in the same straight line, theangle in Aptenjx varying from 150° to 122°, whereas in thecarinates the two bones are at right angles or at an evenacute angle. That this is not a morphological distinction,but is distinctly related to the development of the shoulder 122 STRUCTURE AND CLASSIFICATION OF BIRDS muscles, has been clearly pointed out by T. J. PAEKEE. Hediscovered an approximation to the struthi
. The structure and classification of birds . atite birds has something to do with their loss of the powerof flight. In the ratite birds and in Hesperornis the scapulaand the coracoid are nearly in the same straight line, theangle in Aptenjx varying from 150° to 122°, whereas in thecarinates the two bones are at right angles or at an evenacute angle. That this is not a morphological distinction,but is distinctly related to the development of the shoulder 122 STRUCTURE AND CLASSIFICATION OF BIRDS muscles, has been clearly pointed out by T. J. PAEKEE. Hediscovered an approximation to the struthious condition inseveral of the flightless rails and other birds. But the stateof affairs which characterises the Tubinares warns us againstplacing too much reliance upon this apparently sound gene-ralisation ; for in them we are informed by FOEBES that the angle it (the scapula) forms with the coracoid variesmuch in different genera, being most acute in Pelecanoides,whilst in the Oceanitidge it is hardly if at all less than a right pc FIG. 69.—DEVELOPMENT OF SHOULDER GIRDLE OF CHICK (AFTER LINDSAY). i1/, clavicle ; j>c, procoracoid ; cor, coracoid ; sc, scapula. 1-3, fifth day ; 4, sixth day ; 5, late on sixth day. angle. The widest angle in a carinate bird is 106°, so thereis a difference of only 16° between extremes of carinates andratites. The two clavicles - sometimes spoken of collectively asthe furcula vary much in their degree of development. Theyare totally absent in the Apteryx. In the emu and in certainparrots they are distinct and smallish bones which do notcome into contact with each other ; but in the majority ofbirds they form a single U- or V-shaped bone. The furcula varies in the expansion or non-expansion ofthe base to form a circular hypocleidium. In some birds the 1 On Notornis, in Tr. N. Zealand Inst. xiv. 1882. - A. WEITZEL, Die Furcula : eiu Beitrag zur Osteologie cler Vogel, d. ges. Naturw. xxv. 1865, p. 317. OSTEOLOGY furcul
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