THE SCOTTISH SILURIAN SCORPION. 303 



oiificej so that its prominence slionld offer no obstacle to the 

 act of copulation. 



A short distance behind the genital cleft a similar bnt 

 larger and more conspicuous median cleft is visible. This is 

 flanked on each side by a narrow longitudinally elongate 

 plate or lobe {end., PI. 19), somewhat resembling one half of 

 the genital operculum of recent scorpions. On the outer 

 side of the right-hand lobe lies a bisegmented appendage 

 ['pect., PI. 19), which may be regarded as the homologue of a 

 recent scorpion's pecten or comb. Along the posterior 

 border of this appendage are traceable a number of fine 

 strise occupying the position of the pectinal teeth. Similar 

 strise are traceable upon the left-hand side, although the 

 pecten itself is obliterated. 



Peach regarded the cleft between the two above-described 

 lobes as the generative aperture, a conclusion it is impossible 

 to accept in view of the improbability of the backward move- 

 ment of this aperture on to the somite that bears the pec- 

 tines. The opinion, which I here put forward, that the 

 generative aperture is represented by the slit which, although 

 not mentioned by Peach, appears on his published figure 

 immediately behind the pentagonal prosomatic sternite, 

 seems on morphological grounds far more likely to be correct. 

 Thorell, moreover, suggested that the pair of lobes lying 

 between the pec tines correspond to the small, sometimes 

 longitudinally grooved pectinal sternite of recent scorpions. 

 This may be the true interpretation ; but the shape of the 

 lobes, the length and depth of the groove that separates 

 them, and their relations to the pecten, suggest that they 

 have another significance, and are probably to be regarded 

 as the inner branches of an appendage of which the pecten is 

 the outer branch. From this standpoint the appendage may 

 be compared with the mesosomatic appendages of Limulus, 

 and of the archaic spider Liphistius. In the former the 

 appendages (except in the case of the genital operculum of 

 the Eastern species) consist of a broad foliaceous trisegmented 

 external branch, ;ind of a slender trisegmented internal 



