PTERYGOTUS BANKSII. 73 



of Kington, Herefordshire. It is associated with the Platyschisma helicites and Lingula 

 cornea (Sil. Syst.). These are the two species of shells which accompany the fossils of 

 Lesmahagow above described ; a good argument, therefore, even without other evidence, 

 for regarding these Lesmahagow beds as the uppermost portions of the Ludlow rocks. 1 



The full size must have been from four to five inches long, but the specimens usually 

 met with would probably not be above three or four inches. One or two show the con- 

 nection of the body-rings with the head (PI. XVI, fig. 5) and appendages, or with the 

 tail joints (fig. 4). None are quite complete, and though we have nearly all the parts 

 they are usually disjointed. 



The carapace (PI. XVI, figs. 2 and 3) is a broad semioval ; its length is as six to seven, 

 except when lengthened or shortened by pressure. It is regularly convex, a little pro- 

 duced in front, smooth, and bears the small oval eyes rather more than half way up the 

 head. They are much smaller than in Pterygotm bilobus, being not above one fourth the 

 length of the carapace, and very convex. 



The body is at first wider than the head, and then tapers backwards (PI. XVI, fig. 5). 

 The first ring is very narrow ; the second twice as broad, and with the usual dilated 

 extremities ; the third, fourth, and fifth somites strap-shaped, arched in the middle, and 

 direct on the sides, so that the segment appears much bent. The ends are truncate, in 

 the anterior rings widest behind, and in the posterior ones tapering backwards. 



The hinder rings (PI. XVI, fig. 4) become gradually less transverse, the eleventh only 

 two and a half times wider than long, and the twelfth above once and three quarters its 

 own length. 



The caudal joint (telson) differs, in its expanded form, materially from that of Ptery- 

 gotus bilobus. It is about three fourths as long as wide ; narrow at the base, with two 

 short ridges running down from either angle ; then expanded with somewhat convex sides 

 towards the wide subtruncate apex ; the outer angles are rounded off, the terminal notch 

 shallow, and a short median keel continued from it one third up the segment. 



The sculpture of the head is not known. On the body-rings a transverse lineation, 

 running into open plicae on the sides, occupies the front margin for not quite half the 

 segment ; a few plicae are intermixed with the lines. 3 



(now happily abandoned by Sir R. I. Mnrchison) is altogether inappropriate as applied to the Ledbury 

 rocks. There is not a stone capable of being formed into a tile, from the Downton sandstone to the Corn- 

 stones of Wall Hills ; but there are thin muddy marls over the Downton beds, which would have been tile- 

 stones bad they sufficiently hardened, and which are doubtless the equivalents of the true tilestones. I 

 consider, therefore the term ' passage-rocks,' as used by Sir R. Murchison in the last edition of ' Siluria' to 

 be the more appropriate appellation for these transition beds, and one which allows to the palaeontologist 

 as well as the physical geologist, a broad margin for the line of demar ation between the two great epochs 

 of the Silurian and the Old Red." (' Quart. Journ. Geol. Soc.,' Lond., 1860, vol. xvi, pp. 193—197.) 



1 ' Quart. Journ. Geol.,' vol. xii, 1856, p. 32. See also Part II of this Monograph, pp. 46 — 52. 



2 On a caudal joint (pi. xii, fig. 46, Mem. Geol. Surv., Mon. I) a lineation, parallel to the outer border, 



