Page:The Habitat of the Eurypterida.djvu/258

250 margin is quite as likely to be curved as to be nearly flat (as in S. scoticus). A comparison of the proportions of length to breadth of carapace in these two species and in two others with which relationship might more readily be established will, when taken in connection with the illustrations, show that S. scoticus in so far as its carapace is concerned, is far more nearly related to associated forms in the Old Red and to others in the Ludlow, than to the Shawangunk forms.

From these figures we may conclude that Clarke and Ruedemann find the approximate ratio of length to breadth of carapace in S. myops to be as 2 : 3, but it is evident that in S. scoticus it is 4 : 5. It is not to be denied that the ratio changes from that in the young of S. myops where it is 2 : 3 to that in the type where it is nearly 3:4, and perhaps it might be conceded that in larger forms the ratio might approach 4 : 5; but we cannot be sure. There is in the Ludlow, however, a species which has a carapace proportioned exactly as in S. scoticus and even in the Old Red is a species, S. powriei, with proportions almost the same. Thus there is no need to form conjectures about what might be possible relations to a Middle Siluric species from Appalachia when there are forms which actually show the similarity in formations derived from the same land-mass.

A second point of supposed similarity between S. scoticus and S. myops was the occurrence of long and pronounced epimera in both species. I have in another part of this paper discussed the significance of spinous prolongations on the epimera, but I shall call attention to the arguments again, since they are not universally recognized. Beecher has assembled a wealth of illustration from all branches of the animal kingdom to show that the appearance of spines as a modification of any morphological character marks degeneration in respect to that character, and, when extreme spinosity is accompanied by certain other easily recognizable and similarly degenerate characters the species, genus or family, all members of which show like degeneration, is doomed to decline and extinction. But not only that; as Beecher, Hyatt, and a few present-day palaeontologists, notably Grabau, have shown and have demonstrated by countless illustrations which have led to the most certain deductions, the formation of spines is a homeomorphic character, not in the least indicative of genetic relationship in forms which develop such spines, but marking only an onto- or phylogenetic stage. Spines may and do appear in end-members of totally distinct phyletic groups and are of absolutely no diagnostic value in determining true relations. The Eurypteridae