82 BRITISH FOSSIL CRUSTACEA. 



tracted, for articulation with the previous joint, and has a broad groove running along 

 its whole length. The hinder angles are a little produced. 



Penultimate Segment (figs. 6 to 9, and pi. ix, fig. 15, Mem. Geol. Surv., Mon. I). — 

 This joint is much wider than long, in the proportion of four inches and a half to two 

 inches and a half; some specimens must have been fully five inches long, and, therefore, 

 nine or ten broad. The segment is widest and flattest at the hinder end, the margins 

 are compressed and keeled, except at the thickened and contracted base, and the angle 

 (fig. 9) pointed and produced. The upper surface (fig. 7) is gently convex, but without 

 any ridge, while the lower (?) (fig. 6) has a short thick keel extending half way up. It 

 terminates on the hinder margin of the segment, which is rather deeply notched at this 

 point. The surface is thickly covered with plicae, both above and below, but they are 

 much more prominent on the lower (keeled) surface 1 than on the other, where they are 

 mere surface markings and often obliterated. They cover the whole of the segment, but 

 are less thickly placed towards the hinder margin, at least on the under side (fig. 6). 

 The margin itself is tubercular. 



The squamate keeled lateral borders are ornamented with several (about four or five) 

 rows of oblique thick plicae, more prominent and larger on the lower side ; these are 

 continued from about the anterior fourth of the segment, where the keel commences, to 

 the pointed hinder angle. Similar, but still larger, plicae cover the central keel, and 

 numerous shallow folds run obliquely backwards from the sides to the keel. 



Telson or Tail-joint (pi. ix, figs. 16, 17, Mem. Geol. Surv., Mon. I). — The dimensions of 

 this joint give the best indication of the size to which the species grew; its length was 

 full 5 inches, and the width 4^. The largest specimens of this part in P. anglicus 

 (pi. v, fig. 5, op. cit.) are rather longer, but narrower. The general shape was that of a 

 broad and pretty regular oval, but truncated at the base and emarginate at the apex. 

 The under side is flat, except at the origin of the joint, while it is somewhat convex ; the 

 median line is even concave. Oblique folds or lines, like those on the penultimate segment, 

 occur on the forward half. The upper side is also flattened, but furnished along its whole 

 length with a great central keel, rather thick at its origin, but becoming narrower and 

 more elevated (six tenths of an inch high in the centre), and then decreasing towards 

 the tip. 



Sculpture as in the preceding segment. The blunt ridge of the central keel is 

 covered with small squamate plates, and the margins have four or five rows of oblique 

 elongate ones. The general surface is bare of plicae, except near the base, where they are 

 numerous and prominent both on the upper and under side. 



Appendages. — Chelate A?itennce (pi. ix, figs. 1, 3, Mem. Geol. Surv., Mon. I). — Frag- 



1 We have already stated, in a footnote, p. 80, that in Pterrjgotus there is no keel or ridge to the ventral 

 surface of any segment of the body ; Mr. Salter's description, given above, must therefore be reversed, 

 and his fig. /, pi. viii, op. cit., must be regarded as the under side of the penultimate segment, and fig. 6 

 as the upper or dorsal surface of the body. — H. W. 



