﻿56 BRITISH FOSSIL CRUSTACEA. 



proportion, in this species than in the great PL anglicus (Part I, PI. VIII). The larval 

 eye-spots (ocelli) occupy the centre of the head-shield (see PI. X, Pig. 3, le). 



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Fig. 8 a. 



Fig. 8. 



Fig. 8. Cast of the under side of the head of Pt. bilobus, 1 with the basal joints of the ectognaths (ec) in place ; the large 

 eyes (o, o) are also seen ; Fig. 8 a represents a portion of one of the eyes greatly magnified. 



The Body-segments. — The body, in well-preserved individuals, is barrel-shaped in front, 

 the seven anterior or thoracic segments (8 — 14), taken together, measuring about If inch 

 in length by If in greatest breadth. The thoracic somites in one specimen (PL X, 

 fig. 3) measure 2^ inches in length, but the length of the segments in this example are 

 certainly above the average. 



The anterior segments are extremely narrow in proportion to their breadth, being about 

 five times as broad as they are long. When not crushed or distorted they are gently arched 

 forward in the centre, whilst at the sides the anterior angles are produced forward into 

 blunt lobes, the outer margins being oblique, and their posterior angles a little produced 

 backward, particularly in the seventh thoracic segment (14). This general form of the 

 thoracic segment will be observed in all the species of Pterygoids (see Pis. VIII, X, 

 XI, XII, and XV). 



The following five segments (15 — -19) — which are called abdominal — present a marked 

 change from the preceding, becoming gradually narrower and longer ; the first (15) being 

 three or (at most) four times as wide as long ; the fourth (18) hardly twice as wide as long ; 

 the fifth (19), or penultimate, is nearly equilateral, being only a very little wider than long; 

 both the fourth and fifth somites are a little contracted in front, and have their lateral 

 borders slightly curved and the posterior angles produced ; the fifth somite (19) is cari- 

 nated on its upper side, but only for its lower two thirds. The ' telson' or terminal seg- 

 ment (20) is 11 lines in length by 8 lines in breadth; it is carinated for a short distance 

 at its upper end, whilst at its lower border it is deeply emarginated. 



The outer margins of the body-segments and telson appear to be quite smooth, not 

 serrated or crenated, as in several other species. 



We are unable to give direct evidence of the thickness of the body-segments, as we did 

 in the case of Pt. anglicus (see Part I, pp. 41 and 42) ; but there is little doubt that it 

 was about the same in relative proportion to its bulk. 



1 Copied from pi. i, fig. 4, of ' Geol. Surv. Memoir,' Mon. I, on the Eurypteridce. 



