414 
THE PALEONTOLOGY OF MINNESOTA. 
[Plectambonites sericea 
"Type: Plectambonites planissima Vsmder. Lower Silurian of Russia." (Hall, 
op. cits) 
The following American species are referred to this genus: 
p. ( ? ) decipiens Billings, sp., Calciferous. 
P. (?)sordida Billings, sp., Calcifeious. 
P. sericea Sowerb.v, 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 Sowerhy, sp. 
PLATE XXXII. PIGS. 10-12. 
1839. Leptcena sericea Sowkrbt. Murchison s Silurian System, pi. xix. flgs. 1, 2. 
1840. Strophomena sericea Conrad. Third Annual Report, Geological Survey of New York, p. 201. 
1842. Strophomena sericea Emmons. Geology of New York; Report, Third District, p. 47. 
1847. Leptcena sericea Hall. Palajontology of New York, vol. i, pp. 110, 287, pi. xxxiB, flg. 2: 
pi. LXXIX, fig. 3. 
1852. Leptcena sericea Hall. Ibidem, vol. ii, p. 59, pi. xxi, flg. 1. 
1856. Leptcena sericea Billings. Canadian Naturalist and Geologist, vol. i, p. 41, flg. 2. 
1858. Leptcena sericea Rogers. Geology of Pennsylvania, vol. ii, pt. ii, p. 818. flg. 599. ^ 
1863. Leptcena sericea Billings. Geology of Canada, p. 163, flg. 139. 
1873. Leptcena sericea Meek. Palaeontology 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 sericea Miller. Ibidem, vol. ii, p. 57. 
1875. Leptcena sericea? White. U. S. Geological and Geographical Survey west of the 100th Merid- 
ian, vol. iv, p. 70, pi. IV, flg. 7. 
1883. Leptcena sericea Hall. Second Annual Report, N. Y. State Geologist, pi. xlvi, flgs. 25, 29. 
1890. Leptcena sericea Foerste. Proceedings of the Boston Society of Natural History, vol. xxiv. 
p. 293. 
1892. Plectambonites sericea Hall. Palaeontology of New York, vol. viii, pt. i, pi. xv, flgs. 25-29. 
1892. Leptcena minnesotevsis 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; 
