PTERYGOTUS LUDENSIS. 77 



" (abdominal ? appendages," 1 and possibly a portion of a swimming-foot, together with an 

 imperfect caudal joint. (Mem. Geol. Surv., Mon. I, pi. xii, figs. 4 — 6.) 



The two last may, however, belong to P. gigas, a species very nearly allied, and to 

 which (says Mr. Salter) for some time I believed the whole of the specimens referable. 

 Again, there is so much resemblance in certain points to the Scotch species, P. anglicus, 

 that it requires nice discrimination to separate the three forms. The characters of the 

 antennas, and also of the caudal joint, will, I think, be sufficient. And if subsequent 

 observation should tend to show that this " Tilestone" species is the opposite sex of the 

 P. gigas, it will still have been worth while provisionally to separate them. 



Body -joints. — The complete specimen (PL XVI, fig. 7) shows that the body was not 

 greatly elongated, the segments being all rather widely transverse, the eighth, for instance, 

 being fully four and a half times as wide as long ; the ninth and tenth are gradually 

 narrower, but the eleventh still shows a width two and a half times greater than the 

 length, while in P. anglicus the corresponding joint appears to have been no more than 

 one and a half times the length. 



The penultimate joint is squareish, or rather inversely conical, not much expanded 

 below. It is about one fourth wider than long, as in P. anglicus, and this at the hinder 

 part only. A strong central keel runs down its whole length, covered with large squamae, 

 and the margins are similarly ornamented. (See PL XVI, fig. 8.) If the caudal joint 

 (Mem. Geol. Sur., Mon. I, pi. ix, fig. 18) be of this species, it has lost the terminal 

 apiculus. It is nearly elliptical, the base truncated. It is fully four inches long. 



The sculpture of the body-rings consist of open semicircular squamae, flattened along 

 the anterior border and more convex behind, occupying the anterior half of the segment 

 in the front rings, and more in the hinder ones, till, in the ninth and tenth, they nearly 

 cover the segment. On the sides they are more elongated, and, as in other species {P. 

 gigas, for instance), those on the upper side are more elongate and pointed than those on 

 the lower. All the plicae are prominent and sharp-edged. There are very few interme- 

 diate ones, but the surface of the cuticle is generally roughened between the plicae. 

 Some segments show the plicae very large, and must have been at least fourteen inches 

 broad. Other specimens in the Ludlow Museum show four or five rings overlapping, 

 and some are subcylindrical, and with sharp edges. The caudal joint is broad-oval and 

 shortly apiculated, less abruptly so than in P. anglicus, which, too, has a less regularly 

 oval form, the greatest breadth being below the middle. It is marked all the way dowm 

 dorsally by a strong carina covered with broad squamae, and the edges are also squamate 

 in two or three rows. In the specimen from Trimpley, the sides are marked by oblique 

 radiating interrupted lines. 



The Thoracic plate (" Epistoma and Labrum," Salter) is as large as that of Pt. anglicus, 

 and nearly like it in all its parts. 



1 These are still undeterminable ; they may be cephalic, or thoracic appendages, they are clearly not 

 abdominal ; they probably do not belong to this genus at all : see figures 25 — 27, page 91. — H. W. 



