72 BRITISH FOSSILS. 



The former is destitute of any carina ; on the latter an abbreviated 

 (anal ?) ridge exists, but on the posterior third only (exactly as in 

 P. gigas, Plate VIII. figs. 6, 7). The proportions also of these joints 

 are different from that of the corresponding part in Plate V. figs. 3, 4, 

 being greatly more transverse ; two-thirds as long as wide in fig. 1 , 

 and about three-fifths in fig. 2. This proportion is not very different 

 from that of P. gigas, though the shape is so different. May these 

 not be differences of sex, instead of indicating a second species? 

 The ridge (a), supposed to be anal, is present in all the species, either 

 subcentrally or on the hinder edge of the segment. 



Telson, Plate V., figs. 5, 6. The tail joint (12th) is a broad oval, 

 contracted at its base of insertion, widest posteriorly below the 

 middle, and shortly apiculate at the tip. It is but little carinate on 

 the dorsal view, fig. 5, and flat or with a very slight median ridge 

 below fig. 6. In a specimen six inches long the apiculus does not 

 project half an inch from the general margin, which is squamate and 

 serrate, but not strongly so. The apiculus itself is not serrated. 



Sculpture. 



The whole of the body rings are ornamented with a similar 

 sculpture, although not equal in size or distributed exactly in the 

 same way over all the segments. Curved plicae, like those which 

 occur on the epistoma, occupy the anterior half of each body 

 segment, and are from one to two lines wide, in open semicircular 

 curves upon the middle parts and anterior angles of the segments, 

 but on the front edges they are nearly straight lines and much more 

 crowded. 



This arrangement is nearly uniform for the first five or six rings, 

 the plicae ceasing rather abruptly about the middle of the segment. 

 In the hinder ones they cover a larger surface, Plate V. fig. 1, and on 

 the under sides of all the segments (Plate IV. figs. 5, 6), they also 

 occupy nearly all the surface, though more sparingly placed on the 

 posterior half. On the llth segment, the plicse reach down along 

 the central keel, but are absent from the outer angles, and on 

 the under side, fig. 3 a, they occupy a still smaller area ; while on 

 the caudal joint only a small portion near the base or insertion of 

 the joint is marked by them. 



In the hinder rings, the tenth and the two last especially, the 

 sculpture on the dorsal side is more angular; the plicse being 

 longer and narrower than even on the ventral side, where it is 

 usually more angular than on the upper surface. As inattention 



