DESCRIPTION OF THE SPECIES OF PTERYGOTUS 241 
fig. 12; the epistoma ; a specimen, fig. 1, showing nearly all the body 
joints and telson in position, but a good deal obliterated; the serrate 
bases of the swimming feet, figs. 8, 9; mandibles with palpi, 
figs. 5, 6, 7; antenna, fig. 10; and to these must be added certain 
figures in Plate XII. [Plate 23], showing the antenne, figs. 1, 2; 
the post-oral plate, fig. 3; the bilobed (abdominal?) appendages, 
figs. 4, 5; and possibly the portion of the swimming foot, fig. 6; 
together with the imperfect caudal joint in Plate IX. [Plate 20] 
fig. 18. The two last may, however, belong to P. gigas, a species 
very nearly allied, and to which 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 
antenne, and also of the caudal joint, will, I think, be sufficient. 
And if subsequent observation should tend to show that this “ Tile- 
stone” 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, Plate XIV. [Plate 25] 
fig. 1, shows that the body was not greatly elongated, the segments 
being all rather widely transverse, the seventh, for instance, being 
fully four and a half times as wide as long ; the eighth and ninth are 
gradually narrower, but the tenth still shows a width two and a half 
times greater than the length, while in P axglcus the corresponding 
joint appears to have been no more than one and a half times the 
length (see Plate V [Plate 16] fig. r). 
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 squamz, and the 
margins are similarly ornamented. 
If the caudal joint, Plate IX. [Plate 20] 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 consists of open semicircular 
squame, 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 eighth and ninth, they 
nearly cover the segment. On the sides they are more elongate, and, 
as in other species (P. gigas, for instance, Plate VIII. [Plate 19] 
fig. 5), 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 intermediate ones, Plate XIV. [Plate 25] figs. 2, 
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