503 A. W. GRABAU PALEOZOIC DELTA DEPOSITS OF NORTH AMERICA 



might just as well be considered to be remains of animals caught in the 

 streams by the incoming ocean as the remains of marine forms. The 

 former supposition seems the more likely, since the Eurypterids are not 

 found in the overlying undoubted marine shales and limestones of Middle 

 Siluric or Wenlock age, though they occur in this horizon farther to the 

 north, where it has lost its typical marine character. 



In early Siluric time six species of Pterygotus have been listed by Bar- 

 rande from beds of Xiagaran or Wenlock age (Ee 1? Ee 2 ) of Bohemia, and 

 recently Semper has described three more species and one of Eurypterus 

 from the same horizons. The remains are extremely rare and incom- 

 plete, but the formations they are found in are in part at least undoubt- 

 edly marine. Formtaion ~Ee x is a black graptolite-bearing shale, but these 

 graptolites are found only in certain zones. Ee 2 contains numerous 

 cephalopods, gastropods, trilobites, and corals, and represents in part old 

 coral reefs similar to those in Gotland, to be discussed below. This only 

 serves as another illustration of the imperfect condition in which Eu- 

 rypterids appear when by any chance they are found in a marine forma- 

 tion. 



So far as known, the Wenlock of the Pentland Hills, Scotland, carries 

 the first large Eurypterid fauna of Europe. Sixteen species have been 

 described representing six genera — Eurypterus, Pterygotus, Stylonurus, 

 Drepanopterus, Slimonia, and Bembycosoma. All of the specimens found 

 are in a very fragmentary condition, no perfect individuals having been 

 recorded. The lithological character of the Wenlock of this section indi- 

 cates that in all probability the formation had a delta origin. The Wen- 

 lock is a yellowish sandstone and conglomerate, showing cross-bedding 

 and in some places ripple-marks, and alternating with thin shale beds. 

 This alternation is readily explained in a delta deposit, the shale being 

 subaqueous, the conglomerate and sandstone subaerial or nearly so. If 

 the Eurypterids were marine, they should surely be found in the purely 

 marine equivalents*of these beds, the Wenlock limestones of England, as 

 was also pointed out in connection with the May Hill sandstone, but no 

 trace of them has been reported. The evidence would seem to show that 

 the Eurypterids, or perhaps their molted exoskeletons, were brought down 

 by the streams flowing from the land area of northern Scotland into the 

 sea covering England and southern Scotland, the tests being broken and 

 deposited with the sands and conglomerates in the delta beds. 



Middle Siluric or Salinan. — In the Middle Siluric of North America 

 are several interesting occurrences of Eurypterids and the first appear- 

 ance of well preserved individuals. In the Keefer sandstone at the base 



i 



