The =columellar chain=, which has been already described (p. 157), should be mentioned with the hyoid as it is homologous to the hyomandibular of fishes. The =sternum= of the =Frog=, though regarded as part of the axial skeleton, is so intimately connected with the pectoral girdle, that it will be described with the appendicular skeleton. 2. THE APPENDICULAR SKELETON. This consists of the skeleton of the two pairs of limbs and their respective girdles. It is at first entirely cartilaginous but the cartilage becomes later on mainly replaced by bone. The only bone developed in connection with the appendicular skeleton, which has no cartilaginous predecessor, is the _clavicle_. A. THE PECTORAL GIRDLE. This consists originally of two half rings of cartilage encircling the sides of the body a short way behind the head. These two halves meet one another in the ventral middle line, and separate the anterior elements of the sternum from the posterior ones. Each half-ring bears on the middle of its outer and posterior surface a prominent cup, the =glenoid cavity=, with which the proximal arm-bone articulates. This cup divides the half-arch into a dorsal =scapular= and a ventral =coracoid= portion. The =scapular portion= consists of two parts, the =suprascapula= and the =scapula=. The =suprascapula= (fig. 30, A, 2) is a wide, thin plate attached by its ventral and narrowest border to the scapula. Its proximal and anterior half is imperfectly ossified, its whole border or sometimes only its dorsal and posterior borders consist of unaltered hyaline cartilage, while the rest of it is composed of calcified cartilage. The =scapula= (fig. 30, A, 3) is a fairly stout rod of bone constricted in the middle, and forming the dorsal half of the glenoid cavity. The =coracoid portion= consists of three parts, the =coracoid=, =precoracoid= and _clavicle_. The largest and most posterior of these is the _coracoid_ (fig. 30, A, 4) which like the scapula, is contracted in the middle and expanded at the ends, especially at the ventral end. It forms a large part of the glenoid cavity. The ventral ends of the coracoids which meet one another in the middle line are unossified, and form narrow strips of calcified cartilage, the =epicoracoids= (fig. 30, A, 5); these are often regarded as sternal elements. The =precoracoid= forms a narrow strip of cartilage lying in front of the coracoid, from which it is separated by the wide =coracoid foramen= (fig. 30, A, 9). The dorsal end is continuous with an area of unossified cartilage which separates the coracoid and scapula and forms part of the glenoid cavity. The _clavicle_ is a narrow membrane bone closely attached to the anterior surface of the precoracoid, its dorsal end is expanded. THE STERNUM. The sternum consists of four parts arranged in two groups; two parts to each group. The anterior members are the episternum and omosternum.