SILURIC OR ONTARIO PERIOD 537 



part by C. A. White, Foerste, Prouty, and Kuedemann. The earliest Silu- 

 ric fauna of this trough is contained in the fossiliferous Upper Medina, 

 120 feet in depth, as exposed in the Niagara Eiver gorge. The more im- 

 portant fossils are : Arthrophycus harlani, Dcedalus archimedes, Helopora 

 fragilis, Lingula cuneata, Camarotcechia cf. neglecta, Whitfieldella dblata 

 (these shells are of large growth and far too large to be of Cincinnatic 

 age), Modiolopsis orthonata, M. primigenius, Pleurotomaria (?) perve- 

 tusta, P. (?) littorea, Bucanopsis trilobatus, and Isochilina cylindrica. 

 At Hamilton, Ontario, less than 40 miles from the Niagara gorge, asso- 

 ciated with these fossils in this horizon are others that are clearly Clinton 

 species. This is best shown in the Bryozoa. These strata, and those of 

 Dundas and Flamborough Head, are usually referred to the Clinton, but 

 in the field hold the place of the Medina. In Pennsylvania, Maryland, 

 and Virginia the equivalent horizon is the Tuscarora, or White Medina, 

 sandstone. Ulrich refers the Medina to the Cincinnatic, yet to the writer 

 this fauna seems distinctly younger than anything he has seen referable 

 to the later Eichmond. It is very closely related to the Clinton. On the 

 other hand, if the additional fossils listed by Grabau 155 have been acquired 

 by him, there can be no doubt of the Siluric age of the Medina sandstone. 



The next higher formation of this trough, the Clinton, is marked by: 

 Palceocyclus rotuloides, Monograptus clintonensis, Retiolites genitzianus 

 venosus, Anoplotheca hemispherica, Pentamerus ovalis, Stricklandinia 

 lens, 8. salteri, 8. davidsoni, Brachyprion corrugata, Chonetes, Cornulites, 

 Beyrichia lata, and Calymmene clintoni. 



The higher Magaran strata are marked by Atrypa reticularis, Reticu- 

 laria bicostata, Spirifer crispus, Homoeospira evax, Camarotcechia obtusi- 

 plicata, Dalmanites limulurus, and Homalonotus delphinocephalus. 



Mississippian sea. — On the west side of the Cincinnati axis, in the Indi- 

 ana basin, appear the earliest Siluric deposits of the Mississippian sea, 

 which were first pointed out by Ulrich and Savage. 156 The faunas are 

 clearly of Atlantic origin. In southwestern Illinois, above the Cincin- 

 natic deposits, occurs the Girardeau, with Rafinesquina mesacosta, Lep- 

 tcena rhomb oidalis, Schuchertella missouriensis, Rhynchotreta, Homoeo- 

 spira, Cornulites tenuistriata, C. incurvens, Platyostoma, Strophostylus, 

 Acidaspis halli, Cyphaspis girardeauensis, and Encrinurus. A little 

 higher is the Edgewood zone, with new additions, as Atrypa rugosa, A. 

 putilla, Hindella, Whitfieldella billingsana, Clorinda, Platystrophia, 

 Schuchertella subplanus, Dalmanites dance, and Lichas breviceps clinto- 



155 Grabau : Journal of Geology, vol. 17, 1909, p. 238. 



136 Savage : American Journal of Science, vol. 25, 1908, pp. 431-443. Ibidem, vol. 28, 

 1909 : 516-519. 



