and Clavicular Arch in Sauropterygia. 



139 



may be wedged in the fork between Jie anterior termination of tlie 

 scapulo-precoracoid elements. Or they may be entirely hidden from 

 view, and lie upon the visceral aspect of the scapulo-precoracoid 

 bones. These specimens are all in the Leeds Collection in the British 

 Museum, or in that of Mr. A. N. Leeds at Eyebury. 



They appear to me to show three types of structure : — 

 First, a clavicular arch formed of a large interclavicle with two 

 clavicles forming its lateral wings, joined by squamous overlap, and 

 not by suture. 



Secondly, species in which the interclavicle is a \/" SQa P e d- triangle, 

 and clavicles are doubtfully present. 



Thirdly, species in which two clavicles meet by median suture, 

 without any indication of an interclavicle. 



These modifications are such as might be expected to characterise 

 genera rather than species, and they are accompanied by diversities 

 in other parts of the skeleton. 



FlG. 6. — Part of the shoulder girdle and clavicular arch of Muranosaurus 'platyclis 

 from a drawing by Mr. A. N. Leeds, showing the position of the clavicular 

 arch when found. Co, coracoid ; Sc, scapula ; Into., interclavicle and 

 clavicles. The dark parts are missing. The light shading is a foramen. 



(i.) In the first type the clavicular arch is formed substantially on the 

 same plan as in the Lias genus Plesiosaurus, except that the clavicles 

 rest upon the interclavicle by squamous overlap on its visceral surface, 

 and their posterior-lateral prolongation is broken away. Yet when 

 this surface is compared with that of Plesiosaurus arcuatus an almost 

 identical "]~ _sna P e( i configuration of the interclavicle is exposed, 

 while on the slightly convex ventral surface the clavicles are not 

 seen at all in the specimen as preserved. 



In the skeleton to which this specimen belongs the shoulder 

 girdle is perfectly ossified. The transverse measurement over the 

 humeral articulations is about 16 inches. The median processes of 

 the coracoids are prolonged far forward, so as to make more than half 



