A() BRITISH FOSSIL CRUSTACEA. 
of Dr. Nieszkowski and Professor Hall,’ and corroborates their statements, on the 
evidence of specimens in the collection of Mr. Robert Slimon, of Lesmahagow, Lanark- 
shire.” In a paper published in 1863 I described one of these thoracic plates, which had 
been found by Mr. Slimon, attached to a very perfect specimen of Svimonia acuminata3 
If any uncertainty still existed as to the position of this organ, its discovery in place, in 
Eurypterus, Slmonia, and in two species Pterygotus, ought to preclude all further doubt. 
This plate is well seen, in its normal position, ¢.e. upon the ventral surface of the 
fossil behind the head, and overlying the two first free thoracic somites, in the figure of 
the entire body of Pterygotus, Plate I, fig. 1 ¢; and it can also be discerned on the 
voung specimen figured upon Pl. II, fig. 1. We have represented it in the restored 
figure of this species, at Pl. VIII, fig. 1, op. 
Portions of five of these opercular plates are figured in the Survey Monograph on 
Pterygotus (P1. iu, figs. 2, 3, 4, 5, and 7), but it was at that time assigned to the head, as 
the conjoined epistoma and labrum, being, as before stated, only known from detached and 
imperfect specimens. ‘This plate (which is closely sculptured with the same characteristic 
scale-like markings as those seen upon the body segments, especially at its anterior or 
attached border) is divisible into three well-marked regions: Ist, a narrow, central lobe, 
rounded at its distal free end, and hastate at its proximal end, which is directed forwards, 
and nearly touches the posterior margin of the labium (Plate I, fig. 1 c). Secondly, two 
wide lateral ale, which are united to the sides of the narrow median lobe, and rounded 
off at their lateral and posterior free borders, but are nearly parallel along the anterior 
margin, by which it was, no doubt, attached to the head. The free extremity of the 
median appendage projects beyond the lateral ala, forming together a border very like a 
bracket, thus : 
\a ay 
c 
a a being the lateral ale, and ¢ the projecting median lobe. 
In some specimens a faint indication of a suture is to be discerned on the centre of 
this median appendage, and in Lurypterus lanceolatus* the division is very clearly marked. 
One cannot fail to notice the great resemblance which exists between the thoracic 
plate, or operculum, of these palzeozoic crustaceans and the corresponding plate in the 
recent Limulus (Plate IX, fig. 1, 8, and fig. 1 c). 
Its position is the same in each—being attached to the posterior part of the head- 
shield. In Limulus, the median part is double, being composed of a pair of jointed 
appendages (see Pl. 1X, fig. lc, d,d), but in young Limulus these are anchylosed 
together, as is the case in Lurypterus and Pterygotus.® Professor Hall evidently considers 
1 See Bib., p. 28, ¥ 47. 2 Ibid., p. 25, J 35. 
$ Tbid., p. 29, J 53. 4 Geol. Mag., vol. i, pl. v, fig. 8. 
® See the observations of Prof. Agassiz, in Hall’s Paleeontology of New York, Part vi, vol. iii, p. 395. 
