﻿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 II, fig. 2, a, a), by which it was evidently united to the 

 preceding segment. The other body-rings were, in all probability, joined 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 Pterygotus 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. Ill, 

 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. <T .jjj s* E P imera - 



Sternum. 

 Fie. 6. Section of posterior part of the body of Pterygotus anglicus. ■ 



Mr. Salter, in his description of Pterygotus anglicus} gives a woodcut of a segment 

 extremely like the one figured on PI. Ill, 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 2 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. 



TJie telson, or terminal segment, is obtusely hastate, and strongly keeled upon its 

 dorsal surface. It varies apparently with the age of the individual (see PI. I, fig. 1 t, 

 PI. II, fig. 1, and PI. VI). There are no scale-markings visible on this appendage, 



1 'Mem. Geol. Surv.,' Mon. I, p. 71, fig. 10. 



2 Op. cit., p. 72. 



