516 MR MALCOLM LAURIE ON THE 



When we come to the under surface, the first point to note is the much better develop- 

 ment of the epistoma. This structure (PI. II. fig. 10) was figured and described by Huxley 

 and Salter,* but they were probably misled by the direction of the sculpture on it, and 

 thought that it lay with the straight margin towards the front — a mistake which was 

 corrected by ScHMiDT.t The scale markings on it having their convex side directed for- 

 wards, contrary to the almost universal rule among Eurypterids, would seem to indicate 

 that we have here what is morphologically a portion of the carapace bent over. Schmidt 

 describes this structure as consisting of three pieces ; and in consideration of the beauti- 

 fully preserved and abundant material he has had the opportunity of examining, one is 

 almost bound to accept his description as correct. On the other hand, though some of 

 the specimens I have seen have appeared to support his description, others have been 

 fractured along quite different lines. 



The first pair of appendages, the chelicera? or claws, are well known in Pterygotus. 

 They have been described as consisting of a large number of joints ; but though there are 

 often markings resembling articulations on the proximal portion, yet these show such a 

 complete absence of similarity in different specimens that I believe them to be due to 

 crumplings of the undoubtedly somewhat thin cuticle. These appendages are constantly 

 found detached, and I think they were very likely retractile to a certain extent within 

 the carapace, as are the chelicerse of Thelyphonus among recent forms. If this was the 

 case, there would, of course, be no properly-developed articulation between them and the 

 epistoma, and they would easily become detached. I believe them to have consisted of 

 three joints — a long, straight, proximal one, and the two distal ones, which form the 

 toothed pincers. These appendages, unlike those of Slimonia, were probably prehensile 

 rather than masticatory, and this function may account for the absence of spines on the 

 other limbs, which are purely ambulatory. 



The next four pairs of appendages (PL II. fig. 11) are far simpler and — in proportion to 

 the size of the animal — smaller than in Slimonia, and the first pair seems not in any way 

 different from those following. The basal joint or coxa is, as usual, provided with a row 

 of teeth along its median edge, and these teeth are stronger than in Slimonia. At the 

 posterior angle of the tooth-bearing margin there is a well-developed epicoxite, which 

 may be compared with that of Limulus (PI. II. fig. 12). The rest of the appendage 

 consists of apparently six cylindrical joints, tapering towards the end, and destitute of 

 anything in the way of spines. 



The last pair of feet or ectognaths do not differ in any important respects from those 

 of Slimonia. An exception to the usual simple type of appendage in this genus occurs in 

 Pt. osiliensis,\ in which the joints of the limb are flattened and almost foliaccous, with a 

 single series of spines along one margin. The last pair of limbs in this form also 

 differ from the usual type.§ the terminal joint being smaller and less expanded. This 



* Mem. Geol. Surv., Mon. i. pi. i. fig. i. t Mem. Acad. St Petersb., vol. xxxi. p. 71. 



* Ibid., i. pi. vii. fig. 9. § Ibid., pi. iv. fig. 7. 



