PTERYGOTUS ANGLICUS. 41 
the lateral ale of this plate in Lurypterus to be composed of the sternal pieces of the first 
and second thoracic segments welded together.’ But the thoracic plate, or operculum, in 
the Eurypterida, is formed upon precisely the same plan as in the Xiphosura, namely, by 
the coalescence of the last pair of appendages of the cephalic buckler, and it only hes 
folded close to the under side of the anterior somites. The lateral expansions on each 
side the median appendage are united to it, and really form a part of this pair of modified 
legs, which, in Limulus, bear upon their inner and upper surface a pair of ovaries or 
reproductive organs. 
In Limulus the operculum (Pl. IX, fig. 1, 8, and fig. 1 c) is succeeded by five other 
membranaceous anchylosed pairs of appendages, alike to it in form, but thinner, and 
bearing branchize upon their inner and upper surface. It is reasonable to assume that 
the Lurypterida also possessed these organs, and that, in all probability, the opercular or 
thoracic plate carried the former (7. e. the sexual organs), whilst one or more thinner 
plates, destitute of scale-like markings upon their surfaces, were concealed beneath it, 
as the branchial plates lie hidden beneath the operculum in Limulus, but which are 
extended in the act of swimming. 
The thoracic plate, or operculum, may then be regarded, not as a median appendage, 
like the metastoma, or post-oral plate, but as a pair of coalesced appendages, making seven 
pairs of organs to the head. (See Plate VIII, fig. 1.) 
The body of Pterygotus is seen to be composed of twelve somites or segments, and a 
telson, or terminal joint; the surface of both the upper and the under sides is covered 
with scale-markings, which are more numerous along the anterior half of each segment 
than near the posterior border, which is usually plain. The segments gradually increase 
in width from the head backwards to the fifth segment, when they as gradually become 
narrower to the eighth; the breadth then decreases more rapidly, and the length of the 
segments is considerably increased, until, in the specimen on PI. II, fig. 1, the twelfth 
segment is as long as it is wide. The form of the margins of the anterior segments 1s 
extremely parallel, as well seen in the remarkably fine detached segment figured on 
Plate II, fig. 2, the original of which measures eighteen inches in breadth. 
This was probably about the third segment from the head, and is more than four 
Tergum. 
Epimera. Epimera. 
Sternum. 
Fic. 5. Section of anterior part of the body in Plerygotus anglicus. 
1 Op. cit., 398. 
