xm 



PHYLUM CHORDATA 



417 



cor 



scapular angle is acute ; and there is a furcula. In the Ratitse 

 the coracoid (Fig. 1079, cor.) and scapula (scp.) are much re- 

 duced in proportional size and are ankylosed with one another ; 

 the acrocoracoid (acr. cor.) and acromion (acr.) processes are 

 reduced or absent ; the coraco-scapular angle approaches two 

 right angles ; and there is no furcula, although separate vestiges 

 of clavicles are present in the Emu and Cassowary. In some of 

 the Moas (Pachyornis, &c.) the shoulder-girdle is wholly absent. 

 But, as in the case of the sternum, 

 the distinction is not absolute. In 

 Hesperornis, the Dodo, the Solitaire, 

 Aptornis, Notornis, Ocydromus, and 

 Cnemiornis the bones of the shoulder- 

 girdle are proportionally small, the 

 coraco-scapular angle exceeds 90, 

 and in some cases, such as certain 

 Parrakeets and Owls, the furcula is 

 feeble, or represented by paired 

 vestiges, or absent. Curiously 

 enough, considering that increase in 

 the coraco-scapular angle is usually 

 correlated with diminished powers 

 of flight, it also slightly exceeds 90 

 in the Albatross and some of its 

 allies. 



In most adult Birds the pro- 

 coracoid is reduced to a process on 

 the dorsal end of the coracoid, 

 but in the Ostrich and in the 

 embryo of Apteryx it is well developed and separated by a fenestra 

 from the coracoid. A small bone, the accessory scapula, is some- 

 times found on the outer side of the shoulder- joint. 



The variations in the structure of the wing are mostly matters 

 of proportion, but a remarkable flattening of all the bones is very 

 characteristic of Penguins (Fig. 1074), which are further dis- 

 tinguished by the presence of a sesamoid bone, the patella ulnaris, 

 taking the place of the olecranon process. In the Emu and Kiwi 

 the first and third digits of the normal wing have atrophied during 

 development, the middle one alone remaining. In the Moas 

 (Fig. 1080) no trace of a wing has been found, and in one species 

 only is there even a trace of the glenoid cavity. In the embryos 

 of several Birds an additional digit has been found on the ulnar 

 or post-axial side (Fig. 1081, dg. 4) : this brings the total number 

 of digits up to four, the fifth of the pentadactyle hand alone being 

 unrepresented. 



The simplest type of pelvic girdle is found in Apteryx (Fig. 1082) 

 and the Tinamous, in which both pubis and ischium are free along 



pr.conla 



FIG. 1079. Apteryx mantelli. The 



left shoulder-girdle. A, anterior ; B, 

 lateral (outer) surface, acr. acro- 

 mion ; acr. cor. acrocoracoid ; cor. 

 coracoid ; gl. glenoid cavity ; pr. cor. 

 Ig. procoracoid, reduced to a liga- 

 ment ; scp. scapula. (After T. J. 

 Parker.) 



