lOS 



MAEINE EEPTILES OF THE OXEORD CLaY. 



of the anterior part of the shoulder-girdle is mainly effected by the great increase in 

 size of the ventral bars of the scapulae, which, in the adult, meet in median symphysis 

 and also send back processes which meet the median anterior prolongations of the 

 coracoids. The consequence of this extension of the scapulae inwards to the middle 

 line is, that the clavicular arch comes to lie on the visceral surface of tViose bones and 

 becomes functionless, at least in the adult animal, and undergoes reduction. This 

 may take place in several ways, and there is great variability in the degree to which 

 it is carried, practically all stages from the presence of a well-developed clavicle and 



Text-fiK. 61. 



Shoulder-girdle of: A, Plesiosaurus '? rostrafus (E. 1315, about | nat. size), from below ; 



B, NotJiosauiics sp., from above. (About | nat. size.) 



cl., clavicle ; cor., coracoid ; i.cl., interclavicle ; se., scapula ; v.sc, veutral ramus of scapula. 



interclavicle to the almost complete absence of either or both these elements. Some 

 of the various forms are shown in the text-figures of the shoulder-girdle in the 

 different genera. 



In Murcenosaurus (PI. YI. fig. 3 ; text-figs. 62, 67, 68) the clavicles in most cases 

 undergo great reduction and may become mere paper-thin plates of bone adherent 

 to the inner face of the interclavicle (PI. VI. fig. 3) and of the ventral ramus of the 

 scapula. The interclavicle (PL VI. fig. 6) is usually a well-developed, more or less oval 

 disc of bone, thin at the margin, but thickening towards the middle ; its anterior 

 border bears in the middle line a rounded notch with somewhat thickened edges, 



