orifice, so that its prominence should offer no obstacle to the act of copulation.
A short distance behind the genital cleft a similar but larger and more conspicuous median cleft is visible. This is flanked on each side by a narrow longitudinally elongate plate or lobe (end., Pl. 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., Pl. 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 striæ occupying the position of the pectinal teeth. Similar striæ 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 movement of this aperture on to the somite that bears the pectines. 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 pectines 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, and of a slender trisegmented internal
