﻿40 BRITISH FOSSIL CRUSTACEA. 



of Dr. Nieszkowski and Professor Hall, 1 and corroborates their statements, on the 

 evidence of specimens in the collection of Mr. Robert Slimon, of Lesmahagow, Lanark- 

 shire. 2 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 Slimonia acuminata? 

 If any uncertainty still existed as to the position of this organ, its discovery in place, in 

 Eurypterus, Slimonia, and in two species Fterygotus, ought to preclude all further doubt. 



This plate is well seen, in its normal position, i.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 Fteryyotus, Plate I, fig. 1 c; and it can also be discerned on the 

 young specimen figured upon PI. 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 

 Fterygotus (PI. iii, figs. 2, 3, 4, 5, and 7), but it was at that time assigned to the head, as 

 the conjoined epistomaandlabrum, 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 : 1 st, 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 alee, 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 alas, forming together a border very like a 

 bracket, thus : 



a 



a a being the lateral alee, and c 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 Eurypterus lanceolatus 4, the division is very clearly marked. 



One cannot fail to notice the great resemblance which exists between the thoracic 

 plate, or operculum, of these palseozoic 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 PI. IX, fig. 1 c, d, d), but in young Limulus these are anchylosed 

 together, as is the case in Eurypterus and Fterygotus. 5 Professor Hall evidently considers 



1 See Bib., p. 28, ^47. 2 Ibid., p. 25, f 35. 



3 Ibid., p. 29, ^ 53. 4 Geol. Mag., vol. i, pi. v, fig. 8. 



6 See the observations of Prof. Agassiz, in Hall's Palaeontology of New York, Part vi, vol. iii, p. 395. 



