Manchester Memoirs, Vol.lv. (191 1). No. 16. 5 



shews its natural form, a very rare occurrence. The 

 middle part of the under side of the arch forms an essen- 

 tially flat surface varied by a low median ridge which 

 narrows and falls posteriorly into a spine. If this spine 

 is placed horizontally the arch will slope downward in 

 form, a feature of common occurrence in plesiosaurs. 

 From this central comparatively flat region the lateral 

 wings slope rapidly up at an angle of about 130 with the 

 horizontal : at this bend the bone is about "5 cm. thick. 

 The anterior border is as a whole straight, but there is a 

 central bay some 2'5 cm. wide and rather less than i cm. 

 deep. 



The posterior border of the arch has a median spine 

 which is unfortunately broken off short, but is nearly i cm. 

 thick. The bone on either side of this rod is very thin, 

 and its border runs forwards and then turns gradually out- 

 wards and backwards until it slowly thickens and forms a 

 strong process some 3"5 cm. from the middle line. Out- 

 side this process the border again runs slightly forward and 

 outward until it reaches the outer edge of the bone. The 

 exact shape of this posterior border is quite certain, part 

 of it having been exposed by the splitting of the bone 

 and part developed by myself 



It is not easy to follow the sutures separating the 

 clavicles from the interclavicle in the anterior part of the 

 arch, but posteriorly their relations are quite clear. The 

 interclavicle runs backward as a spine of triangular 

 section, the point of the triangle being downwards, and 

 the clavicles pass backwards leaving a small foramen on 

 each side of the interclavicular spine until they expand 

 into flanges which are tightly attached to the sides of the 

 interclavicular spine. 



The mutual relations of the bones of the pectoral 

 girdle are a^follows : — The corocoids meet in the middle 



