PECTORAL ARCH 143 



former represented by the more constant elements, while the latter 

 tends to become reduced and may even entirely disappear. 

 Clavicles are absent in Chelonians, and are either wanting or rudi- 

 mentary in Crocodiles and Chameleons. 



On the loss of the extremities (certain Skinks, Amphisbaenians, 

 Snakes), the primary shoulder-girdle becomes reduced or even 

 entirely lost, the reduction beginning with the sternum. 



The shifting backwards of the pectoral arch, which is already 

 to some extent seen in Amphibians as compared with Fishes, is 

 still more marked in Reptiles, in which it is situated some distance 

 from the head ; this is especially seen in Chelonians and many 

 fossil forms, and reaches its maximum in Birds. 



In Lizards, unossified spaces are left in the coracoid, giving 

 rise to fenestrae closed over by fibrous membrane. A main fenestra 

 (cf. Fig. 56, a, dorsal to which a bony process, the vestigial pro- 

 coracoid, can be seen) may be distinguished from accessory fenestra? 

 of varied form and number, and is typical of all Lizards: it 

 arises in the primary arch and corresponds to that occurring in 

 Amphibians (Fig. 55). 



Birds. In Birds, the scapula consists of a thin and narrow 

 plate of bone often extending far backwards, the strong coracoid 

 being bent at an acute angle and united by ligament with it in 

 typical Carinate Birds (Fig. 53). In the Ratitse the relatively 

 small scapula and coracoid are ankylosed with one another. 

 The lower end of the latter bone is firmly articulated in a groove 

 on the anterior edge of the sternum, while its upper end takes 

 part with the scapula in forming the glenoid cavity, beyond 

 which it is produced in the Carinatas and in Archeopteryx to form 

 an acrocoracoicl process. 



In Struthio the broad coracoid is fenestrated, and its anterior 

 part may be looked upon as a procoracoid : in other Ratitse the 

 latter is considerably reduced, and may be represented merely by 

 a ligament ; in Carinatae it can often no longer be recognised. 



In almost all Flying Birds the clavicle, a purely dermal bone, 

 is well developed, and becomes united with its fellow to form a 

 furcula (Fig. 53). Amongst the Cursorial Birds, the Emu and 

 Cassowary possess vestigial clavicles: in the others they are 

 wanting, and they have also undergone more or less complete 

 reduction in some Carinate Birds (e.g. certain Parrakeets and 

 Owls). 



Mammals. In Monotremes the pectoral arch retains primi- 

 tive characters, and in them only amongst Mammals does the 

 coracoid extend ventrally to reach the sternum (Fig. 103) ; in all 

 other members of this Class it characteristically becomes reduced, 1 

 and simply forms a prominent process on the scapula (coracoid 



1 In early stages of certain Marsupials (e.g. Trichosaurus), and possibly in 

 all, the coracoid is well developed and articulates with the sternum, but it sub- 

 sequently undergoes reduction. 



