THE GENUS PTERYGOTUS 183 
surface of the animal is exhibited, there lie, on its right side, the 
remains of two appendages, the anterior disposed so that its long 
axis crosses that of the body nearly at right angles, and passing by 
its inner extremity into the middle of the anterior margin of the 
head ; the posterior, lying more nearly parallel with the axis, and 
traceable forwards, only so far as the posterior margin of the anterior 
segment or carapace. 
The latter has a nearly semicircular outline, but is more produced 
than a true semicircle would be. On the right side the margin is 
interrupted by a much elongated oval depression, having its long 
axis directed forwards and inwards. This is the impression of the 
eye, and in some specimens it shows a sculpture very similar to that 
seen upon the eye of P. anglicus. 
The carapace is succeeded by a number of segments, of which the 
best preserved specimens exhibit twelve. The anterior ones are 
transversely, the posterior longitudinally, elongated, and the hinder- 
most, or telson, is oval and deeply emarginate posteriorly. In front 
of the head, in the specimen figured in Plate I. [Plate 12] fig. 1, 
and apparently connected with its anterior boundary, lies a semi- 
circular plate with its convex edge turned forwards (a). The plate 
is as broad as the head, but not quite half so long, and its longitudinal 
axis is oblique to that of the body. In the middle line, it presents 
a flattened area, bounded by a longitudinal ridge on each side, and 
rounded off in front. Although the structure of this part is some- 
what obscure, I have little doubt that it corresponds with the epistoma 
of P. anglicus, and that it has been thrust out of place and thrown 
forward. 
To the right-hand side of the carapace, opposite the anterior half 
of the eye, two curved, serrated, linear impressions, convex inwards, 
appear (fig. 12). The anterior and inner of these is not more than 
half the length of the posterior and outer, and cuts the latter about 
its middle. I find no corresponding impressions upon the left side. 
When the impressions are viewed very carefully with a magnifying 
glass, in a good light, they present minute but important differences. 
The serrations of the short impression are concave and striated, and 
constricted at their bases. Those of the long impression, on the 
other hand, are flattened and not constricted, and their series is 
terminated, posteriorly, by a rounded lobe which forms the posterior 
angle of a broad process whose boundaries become lost externally. 
Sufficient, however, remains to show the correspondence between 
this part and the serrated process of the appendage (c) of P. anglicus, 
while the teeth of the inner and shorter line are strikingly similar to 
