512 MR MALCOLM LAURIE ON THE 



a detached chelicera may be seen at some distance from the animal, but in neither of these 

 specimens is the structure sufficiently distinct to place their existence beyond doubt ; and 

 it was not until I procured a specimen which I could " develop," that I was able to 

 demonstrate their existence and position to my own satisfaction. This specimen I 

 procured from Mr J. Gregory, the well-known dealer in minerals and fossils. It 

 probably came from Tennant's collection, but of this I cannot be sure, and I found after- 

 wards that the other side of the same slab is in the Geological Museum, Jermyn Street. 

 The specimen shows the appendages from the dorsal aspect, the carapace having been 

 shoved off and lying a little distance in front and upside down. The walking legs are not 

 distinctly shown, being much crushed together, and the position of the tactile appendages, 

 lying as they do below the walking legs, can only just be made out. The position of the 

 coxse of the legs, however, is sufficiently distinct, and judicious excavation in front of 

 them, and a little to one side of the middle line, exposed one of the chelicerae (fig. 3). 

 The first joint cannot be distinctly made out, but the second and third joints forming the 

 pincers are quite clear. The pincers are broad at the base, and the two halves are strongly 

 curved, so that they do not meet along their whole length, but only at the points, and 

 there are traces of what may have been teeth along the inner margin of the two rami. 

 They would seem to have lain outside the coxse of the other appendages, as the excavation 

 reached a depth of nearly 4 mm. at their apex. The dimensions of the appendage 

 in my specimen are as follows : — 



Length of second joint of chelicera, . . . .22 ram. 



Length of pincers, . . . . . 10 „ 



Breadth at base of pincers, . . . . . 11 „ 



The discovery of this appendage makes the thoracic appendages agree in number with 

 those of other Eurypterids, and also with those of Arachnids in general. 



The edges of the carapace were bent round on to the ventral surface along the anterior 

 margin, and probably also at the sides. Fig. 4 shows a structure which is found asso- 

 ciated with Slimonia, and probably represents the central part of the epistoma, a structure 

 which is best shown in Pterygotus (v. p. 515). It appears not to have been very strong, 

 as it is somewhat deformed and wrinkled. Whether any ventral sclerites existed between 

 the bases of the legs is not known, but is rendered probable by the form of the coxse of 

 the legs, which would leave a certain space vacant between their attachments to the body, 

 and would seem to need some fulcrum on which to turn. 



When we come to the ventral surface of the free segments of the body, the most 

 conspicuous point at first is that the number of segments appears less by one than when 

 counted on the dorsal side. This is due to the absence of ventral sclerites on the first 

 two segments, their place being taken by the large genital plate or operculum. This 

 operculum consists of a pair of plates and a median lobe (PI. I. fig. 5). These plates 

 are attached by their straight anterior margins so close behind the metastoma that they 

 were originally described as part of it. A triangular area is marked off from the main 

 portion of each plate by a furrow which runs obliquely outwards and forwards from near 



