BRITISH FOSSILS. 17 



singularly close set around the apex of the emargination. It is as 

 if the plate had been once completely oval and evenly ornamented, 

 and had then been folded in at the emargination. 



The part represented in Plate VII. fig. 7, closely resembles the 

 basal joint of the appendage described under the head (6) in general 

 aspect, but differs from it in the excavation of the more convex 

 margin, in the length of the latter, and in the fact that the longest 

 tooth arises considerably behind the junction of the curved with 

 the truncated margin. If these appearances do not arise from 

 mutilation or other alteration, this part is probably a fourth paired 

 appendage. 



The carapace of Pierygotus anglicus has not been found in con- 

 nexion with the body, nor the epistoma with the carapace, nor any 

 of the appendages with any other part of the body. A peculiar 

 value consequently attaches to those entire specimens of a crusta- 

 cean formerly denominated Himantopterus, but which the further 

 information yielded by the. extensive materials on which this 

 Monograph is based, shows to be of doubtful generic distinctness 

 from Pterygotus. 



A well-preserved specimen of P. (Himantopterus) kilobits is 

 represented in Plate I. fig. 1. It presents an elongated body, 

 rounded anteriorly and posteriorly, and much narrower in its pos- 

 terior, than in its two anterior, thirds. Assuming that the dorsal 

 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 anterio 

 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 

 transversly, 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. fig. 1, and apparently 

 connected with its anterior boundary, lies a semicircular plate with 



[1.] B 



