414 THE PALEONTOLOGY OF MINNESOTA. 



[Pleotambonites serloea 



"Type: Pledambonites pianissimo, Pander. Lower Silurian of Russia." (Hall, 

 op. cit.,) 



The following American species are referred to this genus: 



P. ( f ) decipiens Billings, sp., Calciferous. 



P. (9)sordida Billings, sp., Calcifeious. 



P. serieea Sowerby, sp.. Trenton to Clinton. 



P. gibbosa W. and S., Middle Galena. 



P. plicatella Ulrich, sp., Hudson River. 



P. glabra Shaler, sp., Anticosti. 



P. transversalis Wahlenberg, sp., Clinton and Niagara. 



P. transversalis, var. alabamensis Foerste, Clinton. 



P. transversalis. var. prolongata Foerste, Clinton. 



PLECTAMBONITES SERICEA Sowerby, sp. 



PLATE XXXII, FIG8. 10-12. 



1839. Leptcena serieea SOWERBY. Murchison's Silurian System, pi, xix. flgs. 1, 2. 



1840. Strophomena serieea CONRAD. Third Annual Eeport, Geological Survey of New York, p. 201. 

 1842. Strophomena serieea EMMONS. Geology of New York; Report, Third District, p. 47. 



1847. Leptcena serieea HALL. Paleontology of New York, vol. i, pp. 110, 287, pi. xxxiB, flg. 2: 



pi. LXXIX, flg. 3. 



1852. Leptcena serieea HALL. Ibidem, vol. ii, p. 59, pi. xxi, flg. 1. 



1856. Leptcena serieea BILLINGS. Canadian Naturalist and Geologist, vol. i, p. 41, flg. 2. 



1858. Leplcena serieea ROGERS. Geology of Pennsylvania, vol. ii, pt. ii, p. 818, flg. 599. 



1863. Leptcena serieea BILLINGS. Geology of Canada, p. 163, flg. 139. 



1873. Leptcena serieea MEEK. Paleontology of Ohio, vol. i, p. 70, pi. v, flg. 3. 



1874. Leptcena aspera JAMES. Cincinnati Quarterly Journal of Science, vol. i, p. 151. 



1875. Leptcena serieea MILLER. Ibidem, vol. ii, p. 57. 



1875. Leptcena serieea? WHITE. U. S. Geological and Geographical Survey west of the 100th Merid- 

 ian, vol. iv, p. 70, pi. iv, flg. 7. 



1883. Leptcena serieea HALL. Second Annual Report, N. Y. State Geologist, pi. XLVI, flgs. 25, 29. 



1890. Leptcena serieea FOERSTE. Proceedings of the Boston Society of Natural History, vol. xxiv, 



p. 293. 



1892. Plectambonites serieea HALL. Palaeontology of New York, vol. viii, pt. i, pi. xv, flgs. 25-29. 



1892. Leptcena minnesotensis SARDESON. Bulletin of the Minnesota Academy of Natural Sciences, 



vol. iii, p. 329, pi. iv, flgs 24, 25. 



1892. Leptcena prcecosis SARDESON. Ibidem, p, 329, pi. iv, flgs. 26-28. 



1892. Leptcena recedens SARDESON. Ibidem, flg. 330, pi. iv, flgs. 29-32. 



1892. LeptcEna saxea SARDESON. Ibidem, p. 330, pi. iv, flgs. 33-35 



Description: "Shell small, transverse, semioval, approaching semicircular, con- 

 cavo-convex; hinge-line equaling or more frequently a little longer than the breadth 

 of the valves at any point farther forward; lateral extremities varying from some- 

 what acutely angular to nearly or quite rectangular and not properly reflexed; 

 anterior and lateral margins forming together nearly a regular semicircular curve. 



"Dorsal valve concave, its deepest concavity being near the middle; beak not 

 distinct from the cardinal margin; area narrow or nearly linear, and ranging at 

 right angles to the plane of the valves. Interior showing cardinal margin to be 

 minutely [in Minnesota, lower Hudson River specimens, strongly] crenate towards 

 the lateral extremities; cardinal process moderately prominent [smooth] and trifid 

 [simple, the lateral divisions are portions of the incomplete chilidium or crural 

 plates], the middle division being most prominent, with a deep pit at its inner base; 



