12 BRITISH FOSSILS. 



quite distinct when viewed from a little distance, but whose form 

 is by no means easily denned on closer inspection. On the inner 

 side of one of these concavities a broad longitudinal depression 

 traverses the surface for some distance. Primd facie, one would be 

 inclined to take this great plate for a carapace, and the lateral, 

 peculiarly sculptured, concavities for the vestiges of eyes ; and any 

 doubt on the subject will be at once removed on comparing it with 

 the carapaces ofPterygotus Ulobus or perornatus (Plate I. figs. 1, 13) ; 

 but it is somewhat difficult to decide whether the fossil is merely 

 the impression of the dorsal surface of a carapace, or whether we 

 really see the inner face of the carapace itself. The absence of the 

 characteristic sculpture is not conclusive against the former deter- 

 mination, as, with the exception of its anterior edge, the dorsal 

 surface of the carapace of P. perornatus is equally devoid of 

 ornament. 



However this may be, the long margin must clearly be posterior, 

 and the entire lateral edge, the right, while the piece broken off 

 is the left posterior lateral angle. If a line be drawn so as to con- 

 tinue the direction of the anterior part of the left edge backwards 

 until it meets the line of the posterior edge, it will represent the 

 general course of the broken margin, and the carapace will then be 

 found to have had a trapezoidal form, the posterior edge being 

 nearly twice as long as the anterior, and the antero-posterior 

 diameter being about equal to the length of the anterior margin. 

 The right lateral edge is more or less crushed and distorted. 



This is the only .known example* of the carapace of P. anglicus ; 

 the fossil described as such by Professor Agassiz must therefore 

 have a totally different nature. 



2. The Epistoma. It is this remarkable structure which is figured 

 in a state of greater or less perfection in Plate III. figs. 2, 7. 



The most perfect specimens (figs. 2, 5,) reveal a broad plate 

 having one edge nearly straight, but presenting a slight median 

 emargination, while the opposite edge inclines from the middle line 

 on each side outwards towards the straight margin, which it would, 

 if continued, cut at a considerable angle. 



In the middle line a slight longitudinal depression passes along 

 the face of the plate for a short distance, from the straight margin 

 towards the convex one, and then divides into two diverging grooves. 



From the extremities of these, two others, slightly convex 

 towards one another and the median line, are continued as far as 

 the convex edge, and thus the plate becomes divided into three 



* Mr. Salter informs me that there are two more in Lord Kinnaird's collection, which 

 I have not seen. 



