170 



MARINE EEPTILES OF THE OXFOED CLAY. 



Chevrons (text-fig. 6S). — Of the chevrons no good series is available for description. 

 The anterior chevrons (text-fig. 08, A) are of the ordinary Y-shape, the arms being 

 rather shorter than the ventral process ; above they terminate in an oblique rounded 

 facet for union with the facets on the hinder ends of the centra of the caudal vertebras. 

 At the junction of the two arms the bone is compressed from before backwards, but at 

 the lower end the compression is lateral. The exact form and arrangement of the 

 other chevrons cannot be made out from any specimen, but, judging from the skeleton 

 of Geosaums (frontispiece), the condition seems to have been as follows: — As the 

 chevrons are followed backwards towards the bend in the tail they become smaller and 

 the upper arras of the upper Y shorten and widen. At the same time they slope 

 considerably backwards, and a forwardly-directed process arises from the angle of the Y, 

 giving them the appearance shown in text-fig. 08, B, C. Behind the bend of the tail 

 the ventral part of the chevron is greatly enlarged, strongly compressed laterally, and 



Text-fiff. 68, 



Chevron-bones of Melriorhynchus superciliosum : A, anterior chevron from front ; B, posterior chevron from 

 left side; C, from front; D, chevron from end of tail, from left side, and (E) front ; E, posterior 

 chevron from left side. (R. 1530, § nat. size.) 



a.p., anterior process ; /., facet for union with vertebra. 



with its ventral border notched in the middle (text-fig. OS, 1), E). The anterior 

 and posterior ends of the expanded portion bear oval facets apparently for union with 

 the chevrons in front and behind, so that in this region the chevrons form a closed 

 series, much like a row of strongly compressed centra; this arrangement must have 

 greatly strengthened this fin-bearing region of the tail. Towards the extreme end of 

 the tail the ventral portion of the chevron again becomes rod-like and is thickened 

 towards its extremity which terminates in a flat facet (text-fig. 08, F). In Geomuvus 

 the arrangement is very similar (text-fig. A, Introduction), but the expanded chevrons 

 have a convex ventral border without a notch. 



Shouldir-pird/e (text-figs. 09, 70 B). — The shoulder-girdle and fore limb have both 

 undergone great reduction, the latter having apparently been modified to form a 



