M ETRIORI-I YXCHUS. 



157 



being preserved. The plates unite with one another in a complex interlocking 

 suture; they are thickest on their inner edge, which, like the outer, is irregularly 

 serrated; the number of plates in each eye is not known. 



Mandible (PL XI. fig. 3 ; text-fig. 60). — The mandible in Metriorhynchus is very 

 similar in its general structure to that of Steneosanrus, but is distinguished from it by 

 the want of any expansion of the anterior end and by the absence of any lateral 

 vacuity. 



The dentaries (d.) are very large elements. Anteriorly they unite in a symphysis [sym.) 

 to an extent which varies in the different species; in the hinder part of the symphysis 

 they are separated by the wedge-like anterior ends of the splenials (spl.), which are 

 thrust between them to a varying extent. The ventral surface of the symphysis is 

 evenly convex from side to side, and is usually more or less sculptured by numerous 

 grooves and pits ; one groove with vascular pits at intervals, runniug a little below 



Text-fig. 60. 



& 9 n 9- jCOT. 



Mandible oE Metriorhynchus cultridens : A, from outer fide ; B, from inner side. (R. 3804, J nat. size.) 



any., angular ; art., articular ; cor., coronoid ; d., dentary ; p.art., prearticular ; s.any., surangular ; 



spl., splenial ; sym., symphysis. 



the alveolar border, is particularly well marked, especially on the hinder part of the 

 dentary, behind which it is continued along the line of suture between the angular 

 and surangular elements. The oral (upper) surface is smooth and slopes down to the 

 middle line from the ridge which forms the inner border of the row of alveoli. The 

 number and arrangement of these vary greatly in the different species. Posteriorly, as 

 already noted, the inner face of the dentary is overlapped by the outer side of the 

 splenial, while externally it tapers away, lying in a depression in the anterior ends of 

 the angular and surangular. For a short distance along its upper edge it is in some 

 cases overlapped by the anterior end of 1he coronoid. 



The splenials {spl.) form a median symphysis with one another in front ; behind this 

 their inner face is strongly convex from above downwards, and they are much thickened. 



