SILUKIA. 
[Chap. X. 
Five pairs of mouth-appendages are known : — a pair of large 6-7-jointed an- 
tennae with massive chelae, which are furnished with sharp cutting teeth, large and 
small ; three pairs of mandibles with serrate edges, and with a single palpus of 
6 or 7 joints. Then there is the large pair of swimming-feet of seven joints 
each, the basal joint excessively large and serrate at its inner edge, and the 
terminal joints expanded for swimming, like those of the pelagic crabs (Poly- 
bius, &c). 
These are all attached to the carapace. No appendages to the rings of the 
body have been observed ; but attached to the posterior border of the head- 
shield on the lower side, and covering the under surface of the first and second 
anterior segments, there is seen a broad thoracic plate, apparently homologous 
with the opercular plate in Limulus, which covered, no doubt, in the fossil, as 
in the recent Crustacean, the reproductive organs and the branchiae. There 
appears to be good evidence that one or more plates, similar in form, lie con- 
cealed beneath this opercular plate, as in the recent King-crabs. This point of 
structure has been independently noticed by Professor Hall in North America, 
Professor Nieszkowski in the Baltic Provinces, and by Messrs. Salter and H. 
Woodward in this country. (See the Monograph on the Merostomata, Pal. 
Soc. 1866.) 
Eurypterus, though resembling Pterygotus, differs from it in having the 
eyes within the cephalic shield, not upon the border, — also by the want of 
the great pincer-like claws, as we find by the figures given by Professor Hall 
(op. cit.). Eight species of the genus have been found in Britain ; and they 
have similar scale-like sculpturing, though less conspicuous than in Pterygotus. 
The distribution of these two genera is as follows : — 
As already stated, p. 96, there is a rare species of Pterygotus in the Up- 
per Llandovery rocks, and fragments of a Pterygotus occur in the Wen- 
lock beds, though it is only of late that any forms of the genus have been 
found in so old a rock even as the Lower Ludlow, in which two species 
occur : the chief one, distinguished by its prominent and pointed squamae, 
and by its long swimming-feet, has been called Pt. punctatus by Mr. Salter ; 
its mandibles are furnished with large fringed palpi. The other species has 
more open sculpture, like that of the Pt. problematicus, Agas., which is cha- 
racteristic of the Upper Ludlow rock. Of this latter species, mere frag- 
ments are yet known, including parts of the feet, the jaws, the antennae (de- 
scribed as pincers, see p. 237), and some portions of the underside. There 
are, however, at least two large and several small species in the Upper Ludlow 
rock. 
Again, from the Tilestones of Kington, Herefordshire, Mr. Banks has dis- 
interred another fine Pterygotus, which has a wide expanded precaudal joint and 
a head rounded in front. This is not the same species as the large fossil already 
described under the name Pterygotus Anglicus, Agas. ; and, as it appears not 
to be identical with Pt. problematicus of Agassiz, it has been named by Salter 
Pt. gigas *. This must have been truly a gigantic form, rivalling if not 
exceeding in size the nearly allied Pt. Ludensis, already referred to in 
Chap. VII. 
* Geol. Surv. Mem., Monograph, No. I. 1859. cimens from Lanarkshire, he liberally made over 
In justice to Mr. E. W. Banks, of Kington, I must his materials for description in the Geological Sur- 
here state that he had long collected materials vey Memoirs, Monograph No. I. 
for the illustration of Pterygotus gigas, and had Pterygotus Ludensis, so abundant at Ludlow 
been fortunate enough to discover a great number and Kidderminster in the ' Passage-beds,' has 
of parts, which he succinctly described in the the greatest analogy with the Scotch fossil, Pt. 
Quart. Journ. Geol. Soc. vol. xii. p. 97. On the Anglicus, which occurs in the lowest part of 
publication, however, of the more complete spe- the Old Eed Sandstone of Scotland. Pt. pro- 
