THE GENUS PTERYGOTUS LZ 
broken away. The shortest or anterior side is very slightly convex, 
the opposite and parallel longest side is a little concave ; that 
lateral margin which is entire is nearly straight, but is rounded off 
where it joins the posterior edge. The surface of the fossil is slightly 
concave, and exhibits neither sculpture nor distinct granulation, but 
it is marked by a great number of sinuous elevations and depres- 
sions. At the outer extremities of the anterior edge are two large 
and deep depressions of an oval form, having their long axes directed 
obliquely forwards and inwards. 
The surface of each concavity presents a great number of minute, 
but regular and close-set depressions (Figs. 1a, 10), which appear 
quite distinct when viewed from a little distance, but whose form 
is by no means easily defined 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 of Pterygotus bilobus or perornatus (Plate I. [Plate 12] 
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 ofthe characteristic sculpture is not conclusive against the 
former determination, 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 
continue 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—t\t is this remarkable structure which is 
1 Mr. Salter informs me that there are two more in Lord Kinnaird’s collection, which I 
have not seen. 
VOL. II N 
