58 BRITISH FOSSILS. 



are figs. 10 and 11 in Plate II. The former is of the natural size, 

 a young specimen, showing the whole of the body rings ; the two 

 hinder ones are left out for want of room in the plate, but these are- 

 well seen in fig. 11. The rings are arched, and were probably 

 very convex, and both the front and back edges of the segments can 

 be seen in our compressed specimen ; the former as a segment of a 

 circle, the latter as its chord. 



The lateral hinder edges of at least some of the segments are 

 produced (fig. 1 Qg) into short processes, and their margins are acute. 



Six segments appear to be thoracic, and are more transverse 

 than the hinder ones, about three times as wide as long (the front 

 ones still wider), and each is marked at its hinder extremity by 

 two short keels* along the median space, not much raised above 

 the surface, and about a quarter of an inch long. The hinder 

 abdominal segments, of which fig. 10^ is the first, are destitute of 

 these median keels, and gradually longer in proportion to their 

 breadth till the tail-joint is reached. The first segment (a) is only 

 three-quarters of an inch long by one and three-quarters broad ; the 

 second and third (6, c) are much narrower, about nine-tenths long. 

 The fourth measures more than an inch in length ; the fifth one inch 

 and a quarter, but rather less in breadth, and narrowed posteriorly. 

 It is destitute of any central ridge or keel above or below. The 

 tail segment (telson) in this species is a large oval plate, terminated 

 by a long apiculus ; the length without the apiculus is two inches 

 and a half, and with it four inches ; the breadth is nearly one inch 

 and three-quarters, narrowing to the base, which is thickened, 

 (probably cylindrical,) the rest being much depressed. On the 

 upper surface a strong carina arises near the origin of the joint, 

 and continues to the tip of the apiculus. The lower side is flat- 

 tened, or but gently convex at the base. The margin of this tail 

 joint is closely serrate or tuberculate in a double line near the base 

 (like that of the margin of the head), running into a single line of 

 distant long tubercles below and along the apiculus. 



This kind of double tuberculate border is conspicuous down the 

 sides of the head, the tubercles being elongated, while on the 

 arched front border there are three or four rows. 



The ornamentation of the body rings is quite minute, and is 

 but rarely seen in our specimens. It is confined to the anterior 

 border of the segments ; the plicae are small, equal, and rather 

 prominent. 



* The first pair is seen in fig. 10, pushed back over the second, and is the best indi- 

 cation of the boundary of the segment a. 



