II IK PECTORAL GIRDLE. 



603 



Lacertilia. The shoulder girdle is formed as two membranous plates, 

 from the dorsal part of the anterior border of each of which a bar projects 

 (Rathke, Gotte), which is free at its ventral end. This bar, which is usually 

 (Gegenbaur, Parker) held to be independent of the remaining part of the 

 shoulder girdle, gives rise to the clavicle and interclavicle. The scapulo- 

 coracoid plate soon becomes cartilaginous, while at the same time the cla- 

 vicular bar ossifies directly from the membranous state. The ventral ends 

 of the two clavicular bars enlarge to form two longitudinally placed plates, 

 which unite together and ossify as the interclavicle. 



Parker gives a very different account of the interclavicle in Anguis. He 

 states that it is formed of two pairs of bones 'strapped on to the antero-infe- 

 rior part of the prassternum,' which subsequently unite into one. 



Chelonia. The shoulder girdle of the Chelonia is formed (Rathke) of 

 a triradiate cartilage on each side, with one dorsal and two ventral limbs. 

 It is admitted on all hands that the dorsal limb is the scapular element, 

 and the posterior ventral limb the coracoid ; but, while the anterior ventral 

 limb is usually held to be the praecoracoid, Gotte and Hoffmann maintain 

 that, in spite of its being formed of cartilage, it is homologous with the 

 anterior bar of the primitive shoulder-plates of Lacertilia, and therefore the 

 homologue of the clavicle. 



Parker and Huxley (doubtfully) hold that the three anterior elements of 

 the ventral plastron (entoplastron and epiplastra) are homologous with the 

 interclavicle and clavicles, but considering that these plates appear to belong 

 to a secondary system of dermal ossifications peculiar to the Chelonia, this 

 homology does not appear to me probable. 



Aves. There are very great differences of view as to the development 

 of the pectoral arch of Aves. 



About the presence in typical forms of the coraco-scapular plate and two 

 independent clavicular bars all authors are agreed. With reference to the 

 clavicle and interclavicle Parker (No. 468) finds that the scapular end of the 

 clavicle attaches itself to and ossifies a mass of cartilage, which he regards 

 as the mesoscapula, while the interclavicle is formed of a mass of tissue be- 

 tween the ends of the clavicles where they meet ventrally, which becomes 

 the dilated plate at their junction. 



Gegenbaur holds that the two primitive clavicular bars are simply clavi- 

 cles, without any element of the scapula ; and states that the clavicles are 

 not entirely ossified from membrane, but that a delicate band of cartilage 

 precedes the osseous bars. He finds no interclavicle. 



Gotte and Rathke both state that the clavicle is at first continuous with 

 the coraco-scapular plate, but becomes early separated, and ossifies entirely 

 as a membrane bone. Gotte further states that the interclavicles are formed 

 as outgrowths of the median ends of the clavicles, which extend themselves 

 at an early period of development along the inner edges of the two halves of 

 the sternum. They soon separate from the clavicles, which subsequently 

 meet to form the furculum ; while the interclavicular rudiments give rise, on 

 the junction of the two halves of the sternum, to its keel, and to the ligament 



