50 BRITISH FOSSIL REPTILES. 



Trionyx Bakbar^. Owen. Plate 5. 



This species, like the Trionyx Henrici, is most satisfactorily and beautifully repre- 

 sented by an entire carapace in the collection of the Marchioness of Hastings, to whose 

 indefatigable researches in the locality of the Eocene sand at Hordwell Cliff, its discovery 

 is due, and by whose skill, tact, and patience it has been faithfully restored from its 

 original fragmentary state. 



The carapace is more slender in proportion to its length, and deeper or more 

 convex in proportion to its breadth, than in the Tri. Henrici. In this species, as is 

 shown in PI. 6, the breadth is greatest towards the fore part of the trunk ; in the 

 Tri. Barbara this is the narrower part, and increases in breadth towards the middle 



of the carapace {ph). 



The antero-posterior diameter or length of the nuchal plate is greater in proportion 

 to its transverse diameter or breadth, and the arched ridge on its inner surface is 

 less strongly developed than in Tri. Henrici. On the outer surface the smooth 

 anterior border, where the plate would seem as if cut away obliquely to an edge, is 

 more extensive in comparison with the rough, worm-eaten surface in the Tri. Henrici 

 (PI. 6, fig. 3), or those in the nuchal plate of Tri. incramdus (PI. 27, fig. 1, cli). The 

 median part of the anterior border is more deeply excavated, and the lateral borders 

 less deeply dentated in the Tri. Barbara. 



The whole of the posterior border of the nuchal plate is thick, sutural, and is 

 articulated to the first neural plate and the anterior costal plates {pl\); the middle 

 part extending backwards to unite with the neural plate, by which also Tri. Barbara 

 differs from Tri. He^irici. 



The first neural plate is shorter and broader in proportion to the length of the 

 costal plates than in the Tri. Henrici, but presents a similar shape, the sides being 

 parallel, and the posterior angles truncate ; in the three succeeding neural plates the 

 sides converge towards the anterior end, but the posterior angles continue to be cut 

 off. The fifth neural plate is oblong and quadrangular, as in Tri. Henrici (PI. 6). 

 Tn the sixth neural plate the fore part is the broadest, and its angles are truncate ; the 

 seventh is a subtriangular and not fully-developed plate ; the corresponding pair of 

 costal plates meeting behind it. The eighth pair of costal plates {ph) similarly 

 supersede and take the place of s8, by meeting and joining at the middle Hne, but the 

 left is the broadest, not the right. 



The first costal plate (^^li) is longer in proportion to its breadth (or antero- 

 posterior diameter), which is also more equally preserved throughout its length than 

 in the Tri. Henrici, and the connate smooth rib is less prominent on its under surface. 

 The inner and anterior angles of this surface do not show the depression formed by 

 the head of the vertical scapula, which is present in that part of the stronger 

 Tri. Henrici. 



