276 ME. H. M. BEEJS"AED ON THE 



mentary appendages of tlie second segment. It seemed very 

 improbable that the rudimentary appendages of the first segment 

 had fused longitudinally with those of the second segment. 



The actual method of fusing, it seems to me, is made quite 

 clear by the specimen of Tarantula, the operculum of which is 

 here drawn (PL VIII. fig. 1). The conditions there seen may 

 be explained by assuming that the limbs of the first abdominal 

 segment folded together backwards in the median line, as shown 

 in the diagram (fig. 7) ; they thus passed between the rudimentary 

 limbs of the second segment. The large plate of the present 

 genital operculum is thus a composite structure. The anterior 

 and median posterior portions belong to the appendages of the 

 first segment ; the lateral portions are the remains of the limbs 

 of the second segment wbich have been folded back over the 

 stigmatic apertures*. 



The amount of fusion between the two pairs of rudimentary 

 appendages composing the genital operculum is therefore not 

 great. We only require the fold growing backwards from the 

 (? first joints of the) first pair of limbs to fuse on each side of 

 the median line with the inner edges of the limb-buds or pro- 

 minences of the second pair. Anteriorly and laterally, both the 

 rudiments were confluent with the abdominal surface. 



In this way the difficult morphological problem presented by 

 the genital operculum of the Pedipalpi is not hard to solve. It 

 is clearly an acquirement within the Arachnidan phylum, and 

 not, as Laurie claims, a primitive feature inherited from Eury- 

 pterine ancestors. In the first place, the evidence which Laurie t 

 adduces in favour of the existence of a large operculum covering 

 two segments in SUmonia is far from conclusive ; and, in the 

 second place, if it were, it would not necessarily bring the Eury- 

 pterids any nearer to the Arachnids. As Laurie appears to 

 recognize, if such a genital operculum were a primitive feature 

 of the Pedipalpi inherited from Eurypterine ancestors, it would 

 imply that the Arachnids are not a natural group, inasmuch as 

 the genital operculum in all the other important Arachnids is 

 more primitive than it is in the Pedipalpi. Fortunately there 



* I have briefly discussed this method of folding down in "Vestigial Stigmata 

 in the Arachnida," Ann. & Mag. N. Hist. xiv. 1894, p. 149. 



t "The Anatomy and Eelations of the Eurypteridas," Trans. Eoy. Soc. 

 Edinb. xxxvii. (2) 1893. 



