470 CHORD ATA. 



longer protrudes from the surface as a "tail" but may 

 even occasionally become fused to the sacrum. 



The Limb-Girdles.— The girdle of the fore-limb or 

 pectoral arch closely approximates to the reptilian type in 

 the Monotremata^ but becomes more specialised in the 

 Marsupialia and Eutheria. 



In the Monotremata the coracoids are large and meet 

 the sternum. They bear on their inner border a pair of 

 precoracoids. There is also a T-shaped episternum. In 

 the Metatheria and Eutheria the coracoids atrophy, as also 

 the precoracoids. Amongst other vestiges of these bones 

 there is a process upon the scapula, the coracoid process^ 

 which is said to be the distal end of the precoracoid, the 

 true coracoid being represented by a small bone taking part 

 in the formation of the glenoid cavity. 



Hence in nearly all mam- 

 Fig. 324.— Three Types of mals the scapula alone is 

 Mammalian Scapula. left to bear the fore-limb, 



c _ especially as in a great num- 



ber the clavicle atrophies. 

 The scapula is correspond- 

 ingly highly developed. It 

 is a large, triangular-shaped, 

 flattened bone, with a bony 

 ridge down its outer surface 

 called the spine^ terminating 

 in a free process, the acro- 

 mion^ to which the distal end 



A, Cursorial. B, Aquatic or natatorial. r ^^ i • i • ^^ 1 j 



c, Arboreal. of the claviclc IS attached, 



when present. 

 In the running types ( Ungulata), in which the limb has 

 little diversity of movement, the clavicles go and the scapula 

 is long and tapering, with short suprascapular border. In 

 the climbing types {Primates), with varied movements of 

 the fore-limb, the scapula is an approximation to an equi- 

 lateral triangle, whilst in the swimming types (whales, seals) 

 the scapula is broadened out, shortened lengthwise, with 

 long suprascapular border. The spine is pushed forwards, 

 so that the postscapular fossa is very large and the pre- 

 scapular fossa is small. 



