14 BRITISH FOSSILS. 



whether their union was effected by a more definite articulation. 

 In Plate IV. fig. 1, a segment is seen to send a strong process for- 

 ward from its anterior and external angle, and a similar structure 

 appears to have obtained in the specimen figured by Professor 

 Agassiz, in the right-hand upper corner of his Plate A (loc. tit.} 

 This process may have served to articulate the segment with it& 

 predecessor, or it may have given attachment to muscles. 



The specimen figured in Plate V. fig. 2, would at first appear to 

 indicate that the segment had a sub-circular or elliptical section, 

 but it is so much crushed and distorted, that I am not inclined to 

 lay any great stress upon the conclusions which may be drawn 

 from it, the more especially as all the other specimens which give 

 a view of the thickness of the body rather lead to the belief that 

 it was considerably depressed, at any rate, posteriorly. The seg- 

 ment, probably penultimate, figured in Plate V. fig. 3, for example, 

 exhibits no sign of having undergone any very considerable com- 

 pression, and yet its thickness does not equal more than one-fourth 

 of its breadth. It is quite possible, and indeed probable, however, 

 that the proportions of the antero-posterior and transverse diameters 

 may have been different in the anterior and posterior segments. 



It is probable that the segments were sculptured on the ventral, 

 as well as on the dorsal surface, in all cases, as that last referred to 

 certainly is so. No segment has as yet exhibited any trace of an 

 appendage or of an articular surface for one. 



A specimen in the possession of Mr. Lightbody exhibits at least 

 eight segments connected together, but there is no evidence as to 

 the total number composing the body of P. anglicus. 



4 The Telson. Reasoning from the analogy of P. acuminatus, 

 &c., the part figured in Plate V. fig. 6, must be the telson, or ter- 

 minal segment of the body, of P. anglicus. It is particularly 

 described in the systematic portion of this Monograph (p. 72). I will 

 merely remark, therefore, that it is oval, truncated anteriorly, mucro- 

 nate posteriorly, and serrated along the lateral edges. It presents 

 traces of a median ridge, and appears to have been very flat. 



5. The paired Appendages. The remains of three distinct kinds 

 of paired limbs, besides single appendages, have been discovered in 

 the same beds with the carapace and body segments of P. anglicus. 

 These are : 



(a.) The chelate organs figured in the most perfect state in which 

 they have yet been discovered in Plate VI. figs. 4, 5. Three 

 joints at least may be observed in this member. The first is an 

 elongated subcylindrical stem, flattened by pressure. The next, 



