STRUCTURE OF THE VERTEBRATES 163 



2. Scapula. In the reptiles the scapula is a flat, rather nar- 

 row bone. The monotreme scapula is no wider, but the anterior 

 border is turned outward. The embryonic scapula of the marsu- 

 pials is similar, and then develops an anterior extension of the 

 blade of the bone, which leaves the upturned margin as the spine 

 of the scapula. The spine divides the lateral face of the bone 

 into two fossae in which shoulder muscles are attached. The 

 placental scapula is similar, although the development is less 

 diagrammatic. 



3. Clavicle. The clavicle is the most recent and most vari- 

 able bone of the shoulder girdle. It may be small or entirely 

 absent. In the placentals there is a marked correlation between 

 its size and the function of the pectoral limbs. In general terms, 

 the bone is small or absent in running animals; while the 

 climbing types retain a well developed clavicle. When the legs 

 move in a single, antero-posterior, plane the clavicle is a liabil- 

 ity; but when the limbs are moved in many directions the 

 clavicle has selective value for muscular attachments, and for 

 bracing the shoulder. 



G. Pelvic Girdle 



The evolution of the pelvic girdle is more obscure than that 

 of the pectoral, and in the fish and amphibia is much simpler 

 in structure. In the fish and earlier amphibia the anterior limbs 

 carried a greater functional burden. In the fish the pectoral fin 

 is the major balancer of the body, and in the primitive amphibia 

 the head is raised from the ground on the fore limbs as the 

 animal wriggles through the mud. These facts probably explain 

 the greater selection exerted upon the anterior appendages. 



Cladoselache gives the earliest clue to the development of 

 the pelvic region, the fin being supported by isolated cartilages 

 with no body supports. In the sharks the anterior rays have be- 

 come fused into a relatively large cartilage on either side, the 

 two meeting ventrally. Further evolution is indicated by the 

 chondrostean fish, the ventral portion of each cartilage con- 

 stricting off to form a paired ventral segment. In the living 

 crossopterygian (Polypterus) these two segments have fused to 



