BRITISH FOSSILS. 13 



regions, a narrow " median lobe " and two wide " lateral al?e." 

 The inner angles of the alse are rounded off; the median lobe pro- 

 jects beyond them and ends in a sharply defined, well-rounded, free 

 edge. The outer fourth of the transverse diameter of the median 

 lobe on each side is overlapped by the corresponding ala, a con- 

 siderable amount of matrix being occasionally interposed between 

 the two. The general plane of the median lobe, in fact, lies behind 

 that of the alse, whence it arises that impressions of the plate 

 present the aspect figured by M. Agassiz, and reproduced in 

 Plate III. fig. 2, and the median lobe, while covered by the alse, 

 appears spoon-shaped, whereas its end is in reality but little ex- 

 panded (fig. 7). 



The alse present, very well developed, the peculiar sculpture so 

 characteristic of Pterygotus and its allies, multitudes of little semi- 

 lunar facets, with their convex edges raised and all turned towards 

 the convex margins of the plate, being thickly scattered over its 

 surface. 



The facets of the sculpture on the middle of the median lobe are 

 greatly elongated, but have the same general direction. The rounded 

 end of the median lobe is marked by radiating strise, while its sides, 

 where overlapped by the alee, are smooth. From the thinness and 

 sharp definition of its edge, the median process would seem to 

 have been quite unconnected with the alse laterally ; and it appears 

 to have very readily yielded to force along the diverging grooves. 

 The impressions of median lobes detached, in consequence of 

 having broken away from the alse along these lines, are figured in 

 Plate III. figs. 3, 4. 



No crustacean, living or extinct, is known to have a carapace 

 presenting the least approximation to the structure here described, 

 and the true carapace of Pterygotus has already been described. I 

 shall endeavour to prove by-and-bye that this singular plate is in 

 fact the homologue of the epistoma and labrum of other Crustacea. 



3. The Body Segments. Numerous oblong plates and impressions 

 of such, ornamented more or less extensively with the peculiar sculp- 

 ture, have been discovered (Plates IV. and V.). Of these, some are 

 much elongated transversely, and, if we may be guided by the 

 analogy of P. bilobus and perornatus, belonged to the anterior part 

 of the body. Others are longer than they are broad, and it may be 

 concluded, on similar grounds, that they represent posterior seg- 

 ments. In the specimen figured in Plate IV. fig. 6, two such seg- 

 ments remain in their natural relation to one another, but it is, 

 unfortunately, impossible to ascertain whether they simply over- 

 lapped one another and were merely connected by membrane, or 



