464 CHORDATA 



the limb, dividing it into dorsal and ventral portions (fig. 520). The 

 dorsal portion is the scapula (shoulder blade) in the pectoral, ilium in the 

 pelvic girdle. With the development of bone the lower portion is usually 

 split into anterior and posterior parts (fig. 521). The anterior of these is 

 the clavicle in the pectoral girdle, pubic bone in the pelvis; the hinder part 

 is the coracoid or the iscliium in the two girdles respectively. These parts 

 are most constant in the pelvic girdle. In the pectoral girdle either 

 coracoid or clavicle may be lacking, at times both are absent; but no verte- 

 brate with fore limbs lacks a scapula. In the clavicle there is frequently 

 an element, preformed in cartilage, the procoracoid, to be distinguished 

 from a membrane bone, the clavicle in the strict sense. 



In the fishes the girdles are largely or entirely held in position by 

 muscles; in most terrestrial vertebrates there is a more intimate connexion 

 with the vertebral column. In the case of the pelvic girdle the connexion 

 is direct, since the ilium is articulated with one or more sacral vertebra 

 (in reality not with the vertebras themselves, but by the intervention of 

 sacral ribs). The connexion of the pectoral girdle is less direct and is 

 looser. This is effected by clavicle and coracoid. The latter connects 

 with the sternum, which in turn is connected to the vertebral column by 

 the ribs; the clavicle articulates with a bone, the epi sternum, which rests 

 upon the breast bone, the morphological relation of which is doubtful, 

 since under this term have been included different structures. 



Since only the free portions of the appendages are concerned directly 

 in locomotion, and since the various modes of motion swimming, flight, 

 running, leaping, climbing demand special modifications, the skeleton of 

 the limbs shows great variety. It is usually believed that all these forms 

 are to be traced back to an ancestral type, the archipterygium. In this 

 (fig. 520) are numerous skeletal parts which vary little in size and form 

 and are arranged in many closely appressed rows. One of the rows has 

 acquired prominence and is called the principal row; it begins with a 

 larger piece, the metapterygium, which articulates with the girdle and 

 bears either on both sides (archipterygium biseriale} or only on one 

 (archipterygium uniseriale] the lateral rows of skeletal elements. Usually 

 most of the lateral rows are not attached to the principal row, but arise 

 independently from the girdle, and may begin with larger parts, the 

 propterygium and mesoptcr \gium. 



From this archipterygium can be derived a primary form, the penta- 

 dactyle appendage, which serves for all terrestrial vertebrates from the 

 Amphibia onwards (fig. 522). In tracing this from the archipterygium 



following modifications must be supposed. First a reduction in the 

 number of rows to five, a principal row and four accessory rows. The 



