MEASURES OF THE TYNE VALLEY 517 



The first segment bears a structure which, judghig by- 

 analogy with other related forms*, must be connected with 

 the genital orifice. It consists of two rounded ridges close 

 behind the anterior border of the segment and parallel to it, 

 not, however, extending across the whole width of the 

 segment, and seeming to bend forwards at their outer ends. 

 The front ridge of the two is sunk or broken near the middle, 

 but the break may not be a part of its original structure. 

 More doubtful are a pair of structures on the second segment, 

 just possibly marking the position of spiracles. A slightly 

 sunken area on each side cuts into the posterior border of the 

 first segment towards the side of the body, sharply margined 

 in front but with no definite boundary behind. That on the 

 right is much the more definite of the two, the left side of this 

 segment being rather damaged. 



The interpretation of what is seen of the anal plates in this 

 specimen presents several difficulties. The arrangement of 

 these parts that is typical in the Anthracomarti is shown in 

 fig. 10, where a represents the dorsal and b the ventral surface. 

 The plates are marked in this figure in accordance with the 

 views as to their homologies worked out by Pocockf . It will 

 be seen that according to his interpretation the dorsal plate 

 (tergite) of the eighth segment folds round the end of the 

 abdomen between the lateral laminae of the corresponding 

 ventral plate (sternite) ; and that he takes the annular plate 

 encircling the anus to be the ninth segment, and the anal 

 valve (operculum anale) itself to be the tenth|. This reading 

 of the parts shown in the anal region of the Anthracomarti 

 derives some support from the results of Hansen and 

 Sorensen's work on the Cyphophthalmous Opiliones, a group 

 which has been supposed (though not by these authors them- 



* See Fritscli, Palaeozoische Aracliniden, text figiires 41B, 48B, 66A. 



t R. I. Pocook, Eopliiynus and Allied CaAoniferous Araclmida. Geol. Mag., 

 1902, pp. 439, 487; see especially pp. 443-8, and figs. 1 and 2, pp. 490-1. 



% Pocock at first regarded the anal valve in Eophrytms as tlie tergite only of 

 the tenth segment, but later he found reason to believe that both tergite and 

 sternite were represented in it. Sec Geol. Mag., 1903, pp. 250-1, and fig. B, p. 248, 



