HISTOBICAL SKETCH. 



OTHER PUBLICATIONS BEARING ON MINNESOTA, BY MEMBERS OF THE MINNESOTA GEOLOG- 

 ICAL CORPS, ON THE PALEONTOLOGY OF THE LOWER SILURIAN. 



A correlation of the Lower Silurian horizons of Tennessee and of the Ohio and Missis- 

 sippi valleys with those of New York and Canada. E. O. QLRICH, American Geologist, vol. 



I, pp. 100, 179, 303, 333, 1888; vol. II, p. 39, 1888. 



On Sceptropora, anew genus of Bryozoa, with remarks on Helopora, Hall, and other gen- 

 era of that type. E. O. ULRICH, American Geologist, vol. i, p. 228, 1888. 



Preliminary description of new Lower Silurian sponges. E. O. ULRICH, American Geol- 

 ogist, vol. in, p. 233, 1889. 



On Lingulasma, a new genus, and eight new species of Lingula and Trematis. E. O. 

 ULRICH, American Geologist, vol. in, p. 377, 1889; vol. iv, p. 21, 1889. 



Contributions to the micro-paleontology of the Cambro-Silurian rocks of Canada, fart 



II. E. O. ULRICH, Geological Survey of Canada, 1 889. 



New Lamellibranchiata. E. O. ULRICH, American Geologist, vol. v, p. 270, 1890; vol. 

 vi, pp. 173, 382, 1890; vol. x, p. 96. 



New Lower Silurian Ostracoda. E. 0. ULRICH, American Geologist, vol. x, p. 263, 1892. 



Preliminary descriptions of new Brachiopoda from the Trenton and Hudson River gro ups 

 of Minnesota, by N. H. WINCHELL and CHARLES SCHUCHERT, American Geologist, vol. ix, 

 p. 284, 1892. (Distributed April 1, 1892). 



Two new Lower Silurian species of Lichas (sub genus Hoplolichas). E. O. ULRICH, 

 American Geologist, vol. x, p. 271, 1892. 



Geological Survey of Illinois. A. H. WORTHEN, director, vol. vm. Geology and 

 Paleontology, Text and Plates. Edited by JOSUA LINDAHL, July, 1892. American paleozoic 

 Sponges, E. O. ULRICH, pp. 211-251; Descriptions of Lower Silurian Sponges, E. O. ULRICH, 

 pp. 255-282; Paleozoic Bryozoa, E. O. ULRICH, pp. 285-688. 



TABLE OF STRATIGRAPHIC DESIGNATIONS. 



The following table shows approximately the stratigraphic positions of the various 

 terms that have been applied to the different parts of the Lower Silurian in the upper 

 Mississippi valley since 1820. The base of the Hudson River formation has been a well- 

 known horizon, and since its first discovery it has not been changed. The base of the 

 Galena limestone has been a well-known lithologic horizon, and for many years has been 

 accepted as the base of the Galena formation. Owing however to the early enumeration of 

 fossil species which were said to be characteristic of the Galena, in some of the shales and 

 shaly limestones below the principal limestone, by the authors of the term Galena, it was 

 soon found that the Galena formation, on those definitions, must be considered to embrace 

 a portion of the underlying shales. Opinion fluctuated, however, as to the propriety of in- 

 cluding these shales in the Galena, inasmuch as that would destroy the usefulness of the 

 term as a convenient lithological base, and since there was as yet no way of deciding how 



