INTRODUCTORY. LIMB GIRDLES. 25 



dorsally. It is attached by muscle to the vertebral column, 

 and is divided on either side into dorsal and ventral portions 

 by a cavity, the glenoid cavity, at the point where the 

 anterior limb articulates. In higher fishes this hoop is dis- 

 tinctly divided into right and left halves ; it becomes more or 

 less ossified, and a pair of important bones, the clavicles, are 

 developed in connection with its ventral portion. 



In higher vertebrates ossification sets up in the cartilage 

 and gives rise on each side to a dorsal bone, the scapula, 

 and frequently to an anterior ventral bone, the precoracoid, 

 and a posterior ventral bone, the coracoid. The precoracoid 

 is often not ossified, and upon it is developed the clavicle 

 which more or less replaces it. In some forms a T shaped 

 interclavicle occurs, in others epicoracoids are found in front 

 of the coracoids. In all vertebrata above fish, except the great 

 majority of mammals, the coracoids are large and articulate 

 with the sternum. But in mammals the coracoids are nearly 

 always quite vestigial, and the pectoral girdle is attached to 

 the axial skeleton by the clavicle or sometimes by muscles and 

 ligaments only. 



The Pelvic girdle 1 like the pectoral consists primitively 

 of a simple rod or hoop of cartilage, which in vertebrata above 

 fishes is divided into dorsal and ventral portions, by a cavity, 

 the acetabulum, with which the posterior limb articulates. 

 In the pelvic girdle as in the pectoral one dorsal, and (com- 

 monly) two ventral ossifications take place. The dorsal bone 

 is the ilium and corresponds to the scapula. The posterior 

 ventral bone is the ischium corresponding to the coracoid. 

 The anterior ventral bone is the pubis and is generally com- 

 pared to the precoracoid, but in some cases a fourth pelvic 

 element, the acetabular or cotyloid bone is found, and this 

 may correspond to the precoracoid. 



The pelvic girdle differs from the pectoral in the fact that 

 the dorsal bones the ilia are nearly always firmly united to 

 1 See E. Wiedersheim, Zeitschr. wiss. Zool. vol. LIII. suppl. p. 43, 1892. 



