-PTERYGOTUS ANGLICUS. 39 
presence of large and powerful muscles, and we obtain evidence, in the thickness of the 
joints themselves, of the spaces which they formerly occupied. ‘This is well seen in the 
coxognathite, figured on Plate I, fig. 8, which lilts out of its stony matrix, showing the 
opposite side (destitute of ornamentation), the stoutest part being three eighths of an 
inch in thickness. In aged specimens the size of the paddles must have been very great, 
judging from the single articulus which we figure at Plate V. This large penultimate 
joint (prognathite) measures seven and a half inches in length by four and a half inches 
in breadth, and has a dorsal ridge two inches in height, which, no doubt, afforded a 
fulcrum for the attachment of very powerful muscles. 
The Metastoma or post-oral plate——This median appendage is considered by Prof. 
Huxley to be formed by a fold of the integument, and not to represent a distinct somite. It 
is symmetrical in form, and is seen zz szfu on Plate II, fig. 1 m, and Plate VIII, fig. 1 m, 
It no doubt represents the dadcwm, and served more effectually to enclose the posterior 
part of the buccal orifice, being found exterior to the toothed edges of the ectognaths or 
maxillipedes. 
The form of this appendage is heart-shaped in Pterygotus, and was, no doubt, attached 
only by its narrow end to the posterior border of the head upon the under side (Plate III, 
fig. 1 a), with its broader and bilobed end directed forwards (Plate ILI, fig. 1 z). 
. The surface is covered with the same characteristic scale-like markings as those seen upon 
the body-segments, and, like the other plica, they have their convexities directed back- 
wards towards the narrow posterior border (a). The large post-oral plate which we 
figure on Plate III measures five and a half inches in length and four inches in breadth. 
The Thoracic plate, or Operculum.—This_ plate was one of the mutilated portions of 
Pterygotus anglicus first discovered and figured by Prof. Agassiz in 1844,’ which 
(together with other fragments) has been reproduced in Lyell’s ‘ Elements of Geology.” 
A fine series of the detached plates are figured in the ‘Survey Monograph,’ plate in, 
figs. 2—7, but no specimen had at that time been discovered im situ. 
Mr. Page first drew attention to the occurrence of this plate in its natural position 
in Slimonia,*® but considered it to be the “anal plate.” 
Dr. J. Nieszkowski* pointed out the true position of this plate in the Russian species 
of Hurypterus, nm 1858; and in 1862, Mr. Salter’ expressed his concurrence in the views 
1 See Bibliography, ante, p. 22, 4 20. 
2 I observe the reference to these figures in the 6th edition of Sir Charles Lyell’s ‘ Elements of 
Geology’ (1865), p. 523, fig. 590, has by some oversight remained unaltered. The portion of a Thoracic 
plate (I) is referred to the “ back of the head” (as suggested by Prof. M‘Coy), and the fragments 3 and 4 
(the first being part of a maxillipede, or ectognath, and the 2nd the chela of an antenna) still remain united. 
But a reference to the explanation of fig. 591, p. 524, gives the correct position of these several appendages. 
The central ridge, however, seen (in fig. 591) upon the penultimate joint (12) and the telson (13), ought 
not to have been visible, as the figure represents the ventral aspect of the animal. 
3 See Bib., p. 27, 46. 4 Ibid., p. 26, |] 43. 6 Ibid: p. 26, 4 gl. 
