42 BRITISH FOSSIL CRUSTACEA. 
times as wide as it is long. It is strongly arched in the centre, and the body at this 
point was not less than three to four inches in thickness. When seen in section it 
presents somewhat the form represented in the accompanying woodcut. (Fig. 5.) 
Two apodemata are directed forwards on either side from the epimera of the anterior 
border of this somite (Plate IT, fig. 2,@, a), by which it was evidently united to the 
preceding segment. The other body-rings were, in all probability, joimed together by 
similar internal processes from the shelly-envelope, affording solid points of attachment 
for the powerful muscles which doubtless formed—as in the Macrouran Decapod—almost 
the entire bulk of the huge apodal trunk in Péerygotus and its allies. 
As we recede from the head, we find that, at about the seventh, or first abdominal 
somite, the posterior edge becomes more curved, and there is a slight tendency along 
the lateral borders to develop into falcate epimeral extensions. The last three segments 
become slightly keeled along the centre of the tergum, and the border of all the posterior 
segments is somewhat bluntly serrated. 
The very perfect ante-penultimate body-segment which we have figured on PI. III, 
fig. 2, having both its upper and under surfaces preserved, shows the exact thickness 
of this somite to have been three inches: an end view gives a section as under. (Fig. 6.) 
Tergum. 
Epimera. p Epimera. 
Sternum. 
Fie. 6. Section of posterior part of the body of Plerygotus anglicus. 
Mr. Salter, in his description of Pterygotus anglicus,’ gives a woodcut of a segment 
extremely like the one figured on PI. III, fig. 2, showing both the upper and under side. 
The specimen is stated to be from Leysmill, Forfarshire, and may possibly be the same 
example. He calls attention’ to the variation which is noticeable in the form of these 
posterior segments, and suggests that the difference may be due to sex, and not to any 
specific distinction. 
The telson, or terminal segment, is obtusely hastate, and strongly keeled upon its 
dorsal surface. It varies apparently with the age of the imdividual (see PI. I, fig. 1 ¢, 
Pl. II, fig. 1, and Pl. VI). There are no scale-markings visible on this appendage, 
1 «Mem. Geol. Surv.,’ Mon. I, p. 71, fig. 10. 
4 Opicits ps vee 
