130 Basic Structure of Vertebrates 



Fig. 123. The pectoral girdle and fin of Sauripleriis, an upper Devonian crossop- 

 terygian fish. Interest in this type of fin lies in the similarity of relations of the 

 proximal elements of the fin to those found in the pectoral appendage of tetrapods. 

 (After Broom. Courtesy, Neal and Band: "Chordate Anatomy," Philadelphia, 

 The Blakiston Company.) 



otic region of the cranium so that the girdle is anchored to the skull. 

 The pelvic appendage is weakly developed and in some cases merely 

 vestigial. Each fin is supported by a bone lying in the body-wall, but 

 whether this pair of bones represent the girdle or are merely enlarged 

 basal elements of the fin itself is a disputed question. No membrane 

 bones are developed in the pelvic appendage. 



The paired appendages of tetrapods differ from those of fishes 

 mainly in that the freely movable part of the appendage is relatively 

 elongated and more elaborately jointed. The tetrapod girdle, like that 

 of fishes, consists of a ventral and a dorsal part, but with the difference 

 that the ventral part is double, consisting, on each side, of an anterior 

 and a posterior part. A typical tetrapod girdle may be described as 

 three-pronged (Fig. 124). The pectoral girdle consists of a dorsal 

 scapula, a posterior ventral coracoid, and an anterior ventral 

 precoracoid (or procoracoid). The three parts meet at the glenoid 

 facet or fossa where the forelimb is articulated. In the pelvic girdle a 

 dorsal ilium, a ventral posterior ischium, and a ventral anterior 

 pubis meet at the acetabulum where the hindlimb is articulated. 

 Both girdles are preformed in cartilage but, except in amphibians, the 

 adult girdles are largely or completely ossified. In the pectoral girdle 

 the precoracoid undergoes more or less reduction and is functionally 

 replaced by the clavicle (human "collarbone"), a membrane bone 

 which invests the cartilaginous precoracoid (Fig. 125). No membrane 

 bones are developed in the pelvic girdle. The right and left ventral 

 members of the pectoral girdles are usually joined, directly or indirectly, 

 to the sternum. In the pelvic girdle, the corresponding right and left 

 ventral parts usually join one another directly, the junctions being 

 known as the pubic symphysis and the ischial symphysis (Fig. 126). 



