6 BKITISH FOSSILS. 



represented in the right-hand figure of the second row, Plate A,* 

 forbid its arrangement among the Decapoda, notwithstanding the 

 form of the chela figured in the middle of the lower series of the 

 same plate.f I am rather inclined to believe that this singular 

 animal will become the type of a family, intermediate between the 

 Trilobites and the Entomostraca, in which perhaps the Eurypteri 

 and the Eidothece will some day be included. The carapace is 

 ornamented throughout with a squamose sculpture, which confers 

 upon its surface the aspect of a fish's cuirasse ; in the middle it 

 presents an impression like that of a lance-head, corresponding, 

 without doubt, to the gastric and cardiac regions of ordinary Crus- 

 tacea. The divisions of the chela are provided with large obtuse 

 teeth ; the longer point of these levers is strongly arcuated. The 

 arm which carried the chelaj (figured on the left in the lower series) 

 is very large ; the articulations which immediately precede the 

 pincer are short, and wider than they are long. The ordinary legs 

 (figured on the left in the middle series ) are simple, and end in a 

 point ; the segments to which they are attached are sensibly longer 

 than they are wide. 



" The segments of the tail,)] (represented in the right-hand figure of 

 the upper series,) reduced to half their natural size, are broad plates, 

 provided with articular processes at their superior extremities, 

 Lastly, the natatory palettes of the tail (the two figures at the 

 superior left-hand and inferior right-hand corners^]") are rounded 

 plates, fringed along their edges, and scaly on the surface, like the 

 carapace. This curious fossil does not seem to be very rare in the 

 Old Red of Scotland. I have conferred upon this species the name 

 of Pterygotus anglicus, which at once suggests its origin and the 

 singular view which was once entertained as to its nature/' 



Although the parts figured by M. Agassiz have, as I have taken 

 occasion to point out in successive notes, in no single case the nature 

 attributed to them, his sagacious indication of the affinities of 

 Pterygotus deserves thankful recognition. 



The views of Professor Agassiz respecting the affinities of 

 Pterygotus are adopted and expressed still more strongly by M. von 



* This is in fact the epistoma. 



f The part thus figured is one of the antennae. 



| The figure here referred to represents a part of the palp of the ectognath, which has 

 nothing to do with the chela. 



This is an endognath with its palp ; no segment is represented in the figure. 



|| The figure represents one of the most anterior segments of the body. 



j[ The part shown in the superior left-hand figure is part of the basal joint of an ecto- 

 gnath. The inferior right-hand figure represents the terminal joint of the palp of the 

 same organ. 



