248 THE HABITAT OF THE EURYPTERIDA 



is a unique form, and shows a marked divergence from, congeneric 

 forms throughout the world. The telson is elongate, spatulate, with 

 a pronounced median keel, which is represented on the last three 

 segments of the postabdomen as a long spine, rather than a ridge 

 (Woodward, 312, 199, PI. X, fig. 2; 195, p. 35, PI. I, fig. 4). There 

 can be little doubt that this species, which is represented by a single, 

 nearly entire individual, represents a neanic stage of some form, the 

 adult of which probably is not known. Only two and a half inches 

 long, it has the large eyes slightly removed from the border, a feature 

 which is so characteristic of neanic Pterygoti ; but it is difficult to 

 account for the pronounced spines on the body segments, and for 

 the high keel, features, which in associated species are less developed 

 at so early a stage. The shape of the carapace and the position of 

 the eyes suggest P. macrophthalmus from the Bertie, but the spines 

 on the epimera of the last five segments of the postabdomen, the 

 median spine on the last three, the very marked median keel on the 

 telson as well as the proportions of the telson indicate a specializa- 

 tion far beyond that observable in the species just mentioned, par- 

 ticularly when it is borne in mind that all of these features are ob- 

 served in an undoubtedly young individual, which means that they 

 would be much more marked in maturity. This species has all of 

 the appearances of an aberrant form, the relations of which it is 

 impossible to determine from the one known specimen, but it cer- 

 tainly has characters which unite it with Bertie species and with formr 

 which occur in the Baltic region . 



The Stylonuridae of the Old Red sandstone are represented by 

 four species: 6". scoticus, S. powriei, S. ensiformis, and 5. symondsii. 

 The first, represented only by a head and by one nearly entire indi- 

 vidual, is yet so remarkable, so entirely distinct from the typical 

 Stylonurus that it has been set apart by Clarke and Ruedemann as 

 the representative of a new subgenus, Tarsopterus. These two 

 authors have dwelt upon what they consider the close similarity 

 between 6". scoticus and 5. myops from the Shawangunk, stating that 

 "it seems probable, therefore, that 5. myops, when fully known, will 

 prove a representative of the subgenus Tarsopterus of which 5". scoticus 

 is the type" (39, 303). The reasons which they cite are: occurrence 

 of "spurlike epimera of equal relative size," the "outline of cara- 

 pace," and " the approximate position of the eyes and the sculpture 

 of the tergites." Since it is my purpose in the present section of this 

 paper to marshal all of the evidence provided by the relationship 



