ANTERIOR EXTREMITY OF VERTEBRATA. 477 



it is the enlargement of the base of the scapula, which obtains iu 

 the Ohiroptera and Primates. 



§ 360. 



Owing to the development of the primary shoulder-girdle the 

 secondary apparatus, which forms the clavicle (§ 358), is either placed 

 completely iu the background, or used for purposes other than those 

 which it had in Fishes. The Auura only among the Amphibia are 

 provided with a clavicle (Fig. 262, A d), which forms an investing 

 bone for the precoracoid. It is seldom separated from the shoulder- 

 girdle, and this separation is never complete in any forms below the 

 Reptilia (B d). In them it forms a bone which connects the acromial 

 process of the scapula with the episternum (B c). In Birds the 

 clavicle has the same relations ; it is small in Dromasus, and absent 

 in all other Ratita3 ; iu the Cariiiatoe, however, the clavicles soon 

 unite into an unpaired bone, the f urcula, and are connected with 

 the keel of the sternum by ligaments (Fig. 234,/). 



The independent appearance of this portion of the skeleton, the 

 primitive origin of which was that of an investing bone for a piece 

 of cartilage, leads to a histological change in the Mammalia ; the 

 clavicle is in them largely formed from a cartilaginous rudiment, which 

 is similar in many points to all other bones, which are preformed in 

 cartilage. This bone, however, is retained in some Mammals only ; — ■ 

 in those, namely, in which the anterior limbs are capable of a large 

 amount of movement. It disappears so far as to leave no signs of 

 its presence in the Ungulata ; in other forms there are only rudiments 

 of it which are sometimes merely formed by ligamentous bands 

 (Carnivora). 



Anterior Extremity. 

 § 361. 



All the varied forms, which the skeleton of the free appendages 

 exhibits, may be derived from a ground-form which persists in a few 

 cases only, and which represents the first, and consequently the 

 lowest, stage of the skeleton of the fin — the Archipterygium. 

 This is made up of a stem, which consists of jointed pieces of 

 cartilage, which is articulated to the shoulder-girdle, and is beset on 

 either side with rays, which are likewise jointed. In addition to 

 the rays on the stem there are others which are directly attached to 

 the limb-girdle (cf. Fig. 260, d). 



Ceratodus has a fin-skeleton of this form ; in it there is a stem 

 beset with two rows of rays. But there are no rays on the shoulder- 

 girdle. This biserial investment of rays on the stem of the fin may 

 also undergo various kinds of modifications. Among the Dipnoi', Pro- 

 topterus retains the medial row of rays only, which have the form 

 of fine rods of cartilage ; in the Selachii, on the other hand, the 



