PTERYGOTUS ANGLICUS. 35 
head, whilst the remainder are displaced so as to be exposed upon the surface of the shale 
immediately in front of it. 
One of the antennee (c) is well seen, with its large denticulated chela, but only the 
three distal articulations can be discerned. ‘Three gnathites, or jaws, are seen upon the 
right side of the head, and five on the left ; there is little doubt that the two gnathites, 
marked 1, 1, in the figure, although now on opposite sides, both belonged to the right 
side; also one of the detached seven or eight jomted palpi of the gnathites (probably the 
third from the front) on the left side belonged to the right. Figures 1, of the right series, 
and 2 and 3 of the left side, show the gnathites with their palpi attached. The great 
swimming-foot, or maxillipede (5), on the right side, has been twisted round the wrong 
way, and lies with its inner margin turned outwards. ‘The gnathite marked 4, on the 
right side, probably belonged to it. hat on the left side is but little displaced, and 
exhibits the gnathite (5) with its broad rowing appendage still attached. 
The swimming-feet of this specimen exhibit at 7, 7, a small, somewhat triangular plate, 
united to the border of the penultimate joint, and overlapping the oval terminal palette. 
This has not been observed before in Pterygotus anglicus, but I had already detected it in 
P. perornatus, P. bilobus, Slimonia acuminata, and LHurypterus lanceolatus ; and it is 
figured by Prof. James Hall in his history of the American species of Lywrypterus.' 
The impress of the large heart-shaped metastoma, or post-oral plate (v), can clearly 
be discerned in the centre of the head, and the large oval marginal eyes (0, 0) are also 
to be observed on the anterior border. ven the outline of the thoracic plate, or oper- 
culum, is seen beneath the first and second segments of the body. 
The immature condition of this specimen is indicated, not only by its comparatively 
small size (eleven inches in length), but also by the extreme thinness of its integument, 
the whole body being folded or crumpled obliquely across the fourth, fifth, and sixth 
somites, and also across the telson or terminal joint. The indications of a central ridge 
upon the three last abdominal segments is but very slight as compared with the same 
segments in more adult individuals. The telson is much narrower in proportion to its 
length, as compared with the large example figured on Plate VI, whilst its extremely thin 
but prominent central ridge is folded down towards the right side of the median line, 
leaving only a faint mark upon its surface. 
On Plate I, fig. 4, is represented a minute but almost perfect specimen, named 
Pterygotus minor (see Geol. Mag., vol. i, p. 199, pl. x, fig. 2), from the same indurated 
shale as the above example, and in a precisely similar state of preservation. The eyes in 
P. minor ave more within the margin of the head than in the larger specimen (Plate II, 
fig. 1), but we should not place implicit reliance upon this character, as, no doubt, in the 
earliest stage the eyes closely approximated. 
Detached portions and appendages of Pterygotus anglicus—Although for the purpose 
' “Nat. Hist. New York,’ “ Paleontology,” vol. iii, 1859, pp. 397 and 400 
