340 HALL AND SARDESON PALEOZOIC FORMATIONS OF MINNESOTA. 



which enabled them to flourish for hundreds of generations, until their 

 remains had accumulated to the thickness of many feet. The small 

 secluded bay at Saint Croix falls was especially adapted for the swarm- 

 ing of trilobites and lingulas beyond any other spot within the whole 

 Minnesota Potsdam basin* thus far discovered. It would seem from 

 the configuration of this basin that when the accumulations of shells 

 had reached the depth at which currents were felt that the colony dis~ 

 appeared, since the overlying sandstones are epiite destitute of animal 

 remains. . 



The following is the list of fossils known to occur in the Potsdam : 

 Lingula ampla, Owen; Lingidepis pinnseformis, Owen; Obolella politd, 

 Hal] : two species of trilobites and one lingula still undescribed, all from 

 Taylors Falls; other places have thus far shown : 



Lingula mosia, Hall. 

 L. ivinona, Hall. 

 Ob&ella polita, Hall. 

 Orthis pepina, Hall. 

 0. remnichia, N. H.W. 

 0. sandbergi, N. H.W. 

 Bellerophon antiquatus, Whitf. 

 Holopea sweetly Whitf. 

 ■ Aglaspis barrandi, Hall. 

 Agnostus disparilis, Hall. 

 A. josepha, Hall. 

 A. paidis, Hall. 

 Amphion matittinas, Hall. 

 Dicellocephalus lodensis, Whitf. 

 D. minnesotensis, Owen. 

 D. osceola, Hall. 

 D. pepinensis, ( )wen. 

 Ellipsocephalus curtus, Whitf. 

 Illaznurus quadratus, Hall. 



Lonchocephalus chippewansis, Owen. 



L. hamulus, Owen. 



L. wiscpnsensis, Owen. 



Menocephalus minnesotensis, Owen. 



Ptycha sj ris granu losa , < ) wen . 



P. minuta, Whitf. 



P. striata, Whitf. 



Ptychoparia anatina, Hall. 



/'. bidorsa, Hall. 



P. diademata, Hall. 



P. eryon, Hall. 



P. explanata, Hall. 



P. iowensis, Hall. 



P. minuta, Whitf. 



P. oweni, Hall. 



P. perseus, Shu. 



P. shumardi, Hall. 



P. winona, Hall. 



Triarthrella aurunrtis, Hall. 



THE MAGyESIAN SERIES. 



Subdivisions of earlier Writer*. — This complex series, the Lower Magne- 

 sian of Owen, consisting of dolomites, shales and sandstones, was first 

 described by that author in his geological survey of Wisconsin, Iowa 

 and Minnesota."!" Some conception of the complex character of these 



*See Moses Strong, Geology of the upper Saint Croix District: Geology of AVisconsin vol. iii, 

 1880, p. 417 et seq. ; also Warren Upham, Geology of Minnesota, Final Report, vol. ii, 1888, p. 4ns. 

 f I8.VJ, pp. 41-71. 



