POTSDAM PERIOD. 187 



c. Echinoderms. — Sterns of Crinoids, made up of a series of disks, have been 

 found at La Grange, Minnesota (D. D. Owen). They were probably Cystidean, 

 An impression of a single disk has been observed on the sandstone of Keese- 

 ville, N.Y. 



Mollusks. — a. Bryozoans. — No Bryozoans are known of this period, unless 

 some of the Graptolites may be of this nature. (See p. 190.) 



b. Brachiopods. — Fig. 237, Lingula prima Conrad, from Keeseville, N.Y.; 238, 

 same, from Lake Superior (Tequamenon Bay), and from St. Croix, Wis. ; 239, L. an- 

 tiqua H., from St. Croix, — a much larger specimen than those of New York. Other 

 species of Lingulae have been described from the rocks of Wisconsin and Canada. 

 Obolus Apollinis, fig. 236, or a related species, occurs near the mouth of Black 

 River in Iowa (D. D. Owen). Obolus Labradoricus Billings, is a species from 

 the north shore of the Straits of Belle Isle. Obolella Billings, is the name of a 

 related genus of which two species are from the Straits of Belle Isle, one from 

 Troy, N.Y., one from Wisconsin, and one (fig. 244 A) from the Black Hills 

 of Dakota. The genus Discina, or Orbicida (figs. 233, 234), begins in this 

 epoch, and a species is reported by D. D. Owen from the Wisconsin beds, and 



Fig. 244 A. Fig. 244 B. 



Obolella nana. Theca gregarea. 



another, by B. F. Shumard, from Texas. Orthisina festinata B., Camerella an- 

 tiquata B., are other Potsdam species of Brachiopods. 



c. Conchifers. — None are known. 



d. Pteropods. — Fig. 244 B, Theca (Pugiun cuius) gregarea M. & H., from the 

 Big Horn Mountains, lat 43° N., long. 107° W., where they are crowded toge- 

 ther in great numbers on the slabs. Theca primordialis H., from Trempaleau, 

 Wisconsin, and Chippewa River. A Theca has also been found at Keeseville, 

 N.Y. Salterella rugosa B., and S. pidchella B., from the north shore of the 

 Straits of Belle Isle, may be Pteropods. 



e. Gasteropods. — Imperfect specimens resembling a Pleurotomaria and the 

 Ophileta compact a have been observed in Canada, and the former also at Keese- 

 ville, N.Y. A Gasteropod of the form of a Capulus occurs in Texas (B. F. 

 Shumard). 



/. Cephalopods. — Two species of Orthoceras occur in the Potsdam of Canada, 

 in the uppermost layers, along with great numbers of Lingula antiqua (or acu- 

 minata). This discovery by Logan places the Cephalopods lower than they 

 were before known to occur. 



Articulates. — a. Worms.— Fig. 240, casts of worm-holes of Scolithus line- 

 aris H. The Fucoides ? duplex H. (Foster & Whitney's Lake Superior Report, 

 pi. 23) probably belongs to another species of worm. 



b. Crustaceans. — (1.) Phyllopods. — No Phyllopods have been found in the 

 American beds, although they occur in the Primordial rocks of Great Britain. 



