232 THE HABITAT OF THE EURYPTERIDA 



and with one exception between the latter and the Pittsford, is easily 

 understood and entirely to be expected. The baffling break in the 

 phylogenetic history of Stylonurus is also explained. First found in 

 the Pittsford, it came from Appalachia; on that continent its evolu- 

 tion continued through the remainder of Siluric time, its remains 

 not being found because the continental, chiefly river flood-plain, 

 deposits from Appalachia during the Upper Siluric and the Lower 

 Devonic are unknown on the North American continent. The per- 

 plexities which were so detrimental to the "lagoon" theory are com- 

 pletely removed by the river theory. But if the latter be accepted, 

 a new objection arises — only one, to be sure, yet at first it seems to 

 demolish the theory altogether. How does it come to pass that 

 E. pittsfordensis so closely resembles E. lacustris as to seem almost 

 certainly the direct ancestor? In specimens approximately the same 

 size the two species are found to be almost identical in the propor- 

 tions of the cephalon (i.e., length: width = 2 : 3), and in the position 

 and shape of the eyes. On the other hand, the posterior portion of 

 the cephalon flares out in E. pittsfordensis or at least broadens out in 

 a hyperbolic curve, while E. lacustris is marked by the nearly parallel 

 sides of the cephalon. E. lacustris is not so broad a species as E. 

 pittsfordensis, but otherwise does not differ especially in form. The 

 telson in the latter species is unusually long, being nearly equal in 

 length to the rest of the postabdomen. 



An immature, but complete individual in the Buffalo Society of 

 Natural Sciences Museum measures as follows: 4 



mm 



Length of head 21 



Length of body 68 



Length of telson 57 



Total length 146 



In another specimen which is incomplete, the telson measures 

 1 1.5 cm., while in E. lacustris, in an individual of about the same 

 size, it measures only 6.5 cm. In spite of these differences, however, 

 the species are very much alike, though not so closely related as 

 E. lacustris, E. remipes, and E. fischeri, which can be understood from 

 the fact that the three latter belong to the same horizon, while the 

 former precedes them by a long period. I am quite prepared to 

 agree with Clarke and Ruedemann that E. pittsfordensis is the an- 



4 ThesD measurements were kindly furnished to me by Mr. Henry R. Howland. 



