56 BRITISH FOSSIL CRUSTACEA. 
proportion, in this species than in the great P¢. anglicus (Part I, Pl. VIII). The larval 
eye-spots (ocelli) occupy the centre of the head-shield (see Pl. X, Fig. 3, Ze). 
ec EC 
Fie. 8. 
Fig. 8. Cast of the under side of the head of Pt. dilobus,! with the basal joints of the ectognaths (ec) in place; the large 
eyes (0, 0) 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 (s—14), taken together, measuring about 12 inch 
in length by 1? in greatest breadth. The thoracic somites in one specimen (PI. X, 
fig. 3) measure 24 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 arelong. 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 Pterygotus (see Pls. 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 carimated 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 pl. i, fig. 4, of ‘ Geol. Surv. Memoir,’ Mon. I, on the Lurypteride. 
