416 THE PALEONTOLOGY OF MINNESOTA. 
[Plectambonltes gibbosa. 
discussion of Orthis testudinaria, there can be little advantage to the geologist or 
biologist in applying to the numerous local variations of P, sericea specific or vari- 
8tal names. 
Formation and locality.— Common in the Trenton formation of New York, Canada, Pennsylvania, 
central Kentucky, Tennessee, and Dixon, Illinojs. From the "Upper Buff beds'" of the Trenton at 
Rockton, Illinois; Mineral Point, Dodgeville and elsewhere in Wisconsin. In the Trenton shales at 
Minneapolis, St. Paul, Cannon Falls, Fountain and Preston, Minnesota: Decroah and McGregor, Iowa. 
Very abundant in the Galena at many localities in Goodhue and Fillmore counties, Minnesota: Neenah 
and Oshkosh, Wisconsin. In the Hudson River group at Spring Valley, Minnesota; Graf, Iowa; Iron 
Ridge, Wisconsin: Savannah and Wilmington, Illinois; Indiana; Ohio; Kentucky: Anticosti; and Silver 
City, New Mexico. Clinton group of New York. In the Llaadeilo, Caradoc and Llandovery formations 
of England. Also in the Lower Silurian of Scotland, Ireland, Russia, Esthonia, Bohemia, Norway and 
Spain. 
Collectors— C. L. Herrick, W. H. Scofield, E. O. Ulrich and the writers. Also in Dr. C. H. Bobbins' 
collection. 
Mus. Reg. Nos. 193, 228, 793, 3494, 3525, 4054, 4084, 4088, 4090, 5854, 5855, 6747, 6795a, 7929-7939, 7947-7950 
Plectambonites gibbosa W, and S. 
PLATE, XXXII. FIGS. 13-17. 
1892, April 1. Plectambonites gibbosa W. and S, American Geologist, vol. ix, p. 288. 
Shell small, semicircular in outline, strongly concavo-convex, wider than long, 
greatest width along the hinge-line. Surface very finely striate, with six or seven 
stronger lines on each valve, much as in P. transversalis. 
Ventral valve very gibbous and subcarinate medially, lateral slopes rapid and 
slightly concave; greatest elevation about mid-length. Cardinal area strongly ele- 
vated, slightly concave, somewhat wider than that of the dorsal valve; delthyrium 
about as wide as long, with a small deltidium in the posterior portion, and more or 
less occupied by the cardinal process and chilidium of the other valve. Teeth small, 
supported by strong dental lamella?, which join the outer, much elevated margin of 
the muscular area. Diductor muscle pits deep, short, strongly diverging and sepa- 
rated posteriorly by a small septum, on each side of which are slender depressions of 
the adductor scars. Interior otherwise smooth. 
Dorsal valve concave and closely following the curvature of the other valve. 
Cardinal area wide, flat, retrose; delthyrium with a large, simple cardinal process 
more or less covered by a chilidium, which is usually imperfect medially. Crural 
processes short and widely divergent. Adductor scars broadly triangular in outline, 
lobate, with the outer margin strongly elevated. They are separated medially by a 
narrow, deep depression, which is interrupted near the anterior margin of the scai-s 
by a transverse thickening. Two sharply elevated ridges have their origin at the 
hinge-line on each side of the muscular area. These curve much laterally for a 
short distance and then converge, meeting medially quite close to the anterior 
