PECTORAL GIRDLE TETRAPODA 245 



Gegenbaur recognises the archipterygial axis in that of the Tetrapod append- 

 age (fig. 257). The Elasmobranch fin has largely lost its biserial character 

 (best shown in Ceratodus) only a few of the postaxial radials persisting while 

 many preaxial radials remain (fig. 254, F, G). In the transformation of the fin 

 into a podium, a further loss of pre- and postaxial radials and of all basalia 

 except the metapterygium, which forms the stylopodial part of the Hmb. Two 

 of the proximal radials furnish the zeugopodial elements; three of the next 

 radials form the proximal basipodials and the more distal radials furnish the 

 remaining parts of the typical pentadactyle foot, the relations of these being 

 shown by dotted lines in figure 257. A variation of this has the axis passing 

 through the second digit, regarding poUex or hallux as a postaxial ray. 



Another view is that the podial bones have come from the fins of some 

 Crossopterygian Ganoid (fig. 258) where the stylopodial and zeugopodial parts 

 are readily recognized and where a reduction of the distal radials provides for 

 the basipodial parts. Then the fine between the Crossopterygian and the 

 Elasmobranch pterygia is not unsurmountable. 



APPENDAGES OF TETRAPODA 



Pectoral Girdle 



In the lower Tetrapoda the pectoral girdle, as preformed in 

 cartilage, has scapular and coracoid parts, separated by the glenoid 

 fossa with which the hmb is articulated. With the change in the 

 functions of the limb, the girdle differs from that of fishes in details. 

 The two halves either meet in the middle hne or are separated by the 

 sternal structures with which they are connected. The girdle is 

 rarely connected with the axial skeleton directly^ the only connexion 

 being indirect by way of the sternum and ribs. Except for this, 

 the girdle is bound to other skeletal parts only by muscles and 

 ligaments. 



At most there may be the following bones in the pectoral girdle^ 

 — scapula, suprascapula, coracoid, precoracoid and epicoracoid of 

 cartilage origin, while the only membrane bones are the clavicle 

 (cleithra are lost above fishes, unless they persist in some Stegocephals 

 and Cotylosaurs) and the episternum. 



1 Some Plesiosaurs are said to have the scapula connected with the anterior vertebrae; 

 Pteranodon has it articulated with the neural spines of the fused anterior vertebrae. 

 If the precoracoid of the Stegocephala be, as has been suggested, a supracleithrum, there 

 may have been a connexion in this order. Crocodiles sometimes have the scapula 

 attached by Hgament to the first thoracic vertebra. 



^ Basing the argument on the fact that there are two coracoidal ossifications in 

 mammals, the attempt has been made to show other homologies between the mammalian 

 and reptilian girdles than those usually recognized and which are adopted here (see 

 p. 229). 



