ANATOMY AND RELATIONS OF THE EURYPTERID^E. 521 



which is characteristic of Eurypterus, seems to be more advanced and further removed 

 from the Trilobite type than forms like Pterygotus, in which the distinction between 

 body and tail segments is not distinctly marked. The only Silurian Eurypterus known 

 to me in which the distinction between body and tail is not well marked is the little 

 E. conicus ; * and this form, which furthermore has the eyes remarkably near the margin 

 of the carapace, may very probably prove to be the young of some of the larger forms 

 from the same locality. On the other hand, it may be said that Stylonurus, which is 

 almost certainly derived from the Eurypterid stem through forms related to Dolichopterus, 

 or more likely Drepanopterus, has much the same generalised form of body as Pterygotus. 

 The limbs of Stylonurus, however, are highly specialised. 



Another argument which has influenced me in favour of Pterygotus, as representing 

 the form most nearly related to the primitive Eurypterid, is drawn from the appendages. 

 The first pair is, it is true, remarkably different from the common type in the family, as 

 shown in Slimonia and Eurypterus, and has probably been independently modified, while 

 the last pair have no claim to being primitive over those of the same two genera. The 

 four pairs of appendages, however, which lie between these extremes seem to me to yie]d 

 a very strong argument in favour of my view, inasmuch as they are all alike, and all 

 simple in construction, without any elaborate development of spines. In Slimonia the 

 second pair of appendages are highly modified, apparently for a tactile function ; while in 

 Eurypterus, and still more in Stylonurus, the fifth pair differ markedly from the second, 

 third, and fourth pairs. A further point is the apparently much greater development of 

 the epicoxite — a structure common to the Eurypterids, Limulus and Scorpio, and there- 

 fore probably primitive — in Pterygotus than in the other genera. A further argument 

 for placing Pterygotus below Slimonia and Eurypterus is the lesser degree of develop- 

 ment of the median lobe of the genital operculum, though, perhaps, the details of 

 this structure are hardly sufficiently well known to admit of our attaching very much 

 morphological value to it. 



Whether the bilobed telson, which characterises the majority of Silurian Pterygoti, is 

 to be regarded as more primitive than the pointed form or not, must remain for the pre- 

 sent an open question. I think, however, that the balance of evidence is against so 

 regarding it, especially if one considers how characteristic of the earlier Trilobites a 

 pointed telson appears to be. Geological succession gives us no clue to this question, 

 because, while the bilobed forms are certainly the most numerous in the Silurian, never- 

 theless we have, in America at all events, contemporaneous forms with pointed telsons.t 

 It might be argued that, while the advantage of having a weapon at the end of the tail 

 is manifest, it is difficult to see what is to be gained by substituting a bilobed telson for 

 the pointed form ; but we know far too little of the conditions under which these 

 creatures lived for such an argument to have much weight. The only advantage which 

 occurs to one as possibly appertaining to the bilobed form of telson is its greater 

 efficiency as a swimming organ. 



* Trans. Boy. Soc. Edin., vol. xxxvi. t Pohlmann, Bull. Buff. Soc. Nat. Sci., vol. iv. ; and Grote, ibid., vol. iii. 



