ANATOMY AND RELATIONS OF THE EURYPTEEID^. 513 



the base of the median lobe to the anterior margin of the operculum. The other two 

 sides of this triangular area are bounded respectively by the anterior margin and the 

 suture which runs down the middle line. These two triangular areas may represent the 

 paired sternite of the first abdominal segment, the remaining portions of the plates 

 representing the appendages. If this is a correct interpretation, the appendages must 

 have been very firmly attached to this sternite, as I do not remember to have ever seen 

 a fracture along this line. The outer and posterior margins of the plates are strengthened 

 by a thickened border. The median lobe is undoubtedly genital in function, and appears 

 in two distinct forms. In the first of these (fig. 5) the organ terminates at its free end in 

 three sharp points. This form is considered by Dr Woodward to belong to the female, 

 as he has found it associated with the eggs [Parka decipiens). This association appears not 

 very conclusive, as the remains of these organisms are often crowded together very closely. 

 The other form of median lobe (PL II. fig. 8) terminates in a more or less truncated cone 

 which is marked by two or three deep furrows, which appear to me due to its having 

 been eversible. The difference in the number of furrows and in the form of the end 

 would in this case be due to the different extent to which it was protruded in different 

 cases. Dr Woodward's interpretation of this structure, however, is different.* He thinks 

 that there are three similar plates, each with a median lobe lying one on the top of 

 another, the end of each projecting a little beyond that of the one above it. His further 

 arguments for the existence of more than one genital operculum are based on the 

 presence on one slab of two opercula of the first type lying close beside one another along 

 with what may be a portion of a third. Further, some of the specimens show scale 

 markings on the surface of the plates, while others do not, and he suggests that these 

 markings were only present on the uppermost plate. My reasons for dissenting from his 

 views are as follows : — In the first place, the structure is almost certainly connected with 

 reproduction, and it does not seem to me probable that a reproductive organ would be 

 repeated two or three times in forms so highly organised as the Eurypterids. Secondly, 

 if there were three plates, as Dr Woodward suggests, two of them must have 

 been attached to the same segment, as only two segments are present in the portion 

 of the body covered by the operculum. Thirdly, it seems to me very unlikely that 

 the three plates and median lobes should fit so accurately as to show no sign of their 

 existence, except at the apex of the lobe, even in obliquely crushed specimens. If my 

 view that the genital operculum is single is correct, it follows that the presence of two 

 specimens of it on the same slab is purely a coincidence. This is not so improbable as 

 might appear at first sight, for Slimonia seems to have been gregarious, one slab in 

 Dr Hunter's collection showing six or seven large specimens lying inextricably mixed 

 within a space of less than four feet square. The preservation of markings on the remains 

 of these animals seems to me to depend so much on the details of fossilization, and 

 perhaps also on the condition of the animal at death, that their presence on some 

 specimens, and absence on others, is not of much weight as an argument. 



* Loc. cit., p. 116. 



