ANATOMY AND RELATIONS OF THE EUftYPTERID^. 511 



but it seems always present, and is one of the most characteristic structures of the group. 

 The basal joints (coxse) of the last pair of legs — the " ectognaths " or " swimming feet," — 

 which lie immediately under the margins of the metastoma, are retort-shaped, and armed 

 with teeth along what would be the broken-ofF neck of the retort. They constitute the 

 most powerful members of the five pairs of biting organs with which the animal was 

 furnished. To the postero-external angle of the coxa is attached the leg, which consists, 

 as far as I could ascertain, of six joints, the first four of which are short, and articulate 

 somewhat obliquely with each other. The penultimate joint is long and rectangular, and 

 bears at its distal end the oval terminal joint, and also, towards the inner margin, a small 

 sub-triangular piece, which is probably a modified spine. 



This appendage is always described as a swimming organ, but I am inclined to doubt 

 the correctness of this interpretation of its function. The Eurypteridse appear to me, 

 from their general build, more fitted for crawling than swimming ; and I am inclined to 

 explain this appendage as having been used by the animal to get a firm hold on the 

 bottom, and probably also for digging out sand and covering itself, in much the same 

 way that Portunus uses its very similar pair of appendages. 



The three legs next in front of the ectognaths resemble each other very closely, and 

 need not be described separately. Fig. 1 may be taken as typical of them. Here we 

 have again the basal joint with teeth along its inner margin and a six-jointed appendage, 

 the joints of which bear spines attached to the anterior margin. The only hitherto unde- 

 scribed point is the presence of a small process, articulated at the posterior end of the 

 tooth-bearing edge (fig. 1, epc). Whether this structure, which corresponds to the 

 epicoxite of Limulus (fig. 12, epc) and Scorpio, existed on all the appendages I am unable 

 to say, as it is only visible in exceptionally well preserved detached appendages. 

 Anterior to these three pairs of crawling legs or " endognaths " comes a pair which appear 

 to be specially modified for a tactile function (fig. 2). This pair of appendages, first 

 described by Dr Woodward, and termed by him " antennae," are undoubtedly post-oral 

 in position, as the basal joint is armed with teeth. To the basal joint is attached an 

 elongate sub-triangular second joint, which is succeeded by four sub-cylindrical joints, 

 which gradually diminish in width. This makes the appendage consist of six joints in 

 all. Dr Woodward describes and figures it as consisting of eight joints, but I think he 

 has been misled by crumplings of the surface. In its normal position this appendage lies 

 directed backwards across the others (fig. 9), and is on this account not easily seen in 

 situ. It resembles very closely the corresponding pair of appendages in Phalangium, and 

 probably had the same tactile function. In other recent forms, e.g. Thelyphonus, a similar 

 function is performed by the third pair of appendages, and it is possible that these 

 appendages in Slimonia may prove to be also the third pair. 



The most anterior pair of appendages, corresponding to the pincers of Pterygotus, has 

 hitherto not been described. It is preoral in position, and consists of a small pair of 

 chelicerse (PL I. fig. 3). This pair of appendages is indistinctly shown in a specimen in 

 the British Museum, and in one specimen in the collection of Dr Hunter of Braidwood 



