BKACHIOPODA. 453 



Orthis (Dalmanella) amu-ua.] 



Vairety circularis is distinguished by its subcircular outline, usually smaller size 

 and very fine, equal and more numerous striae, a greater number of which terminate 

 on the cardinal line. The tubulose character of the striae is also developed, but it 

 is never a conspicuous feature. Occasionally specimens will be found with very fine 

 striae, which are, however, larger and wider than is usual in this variety. 



0. subcequata, var. circularis, attains its maximum development in individuals in 

 the "Glade limestone" in Tennessee, and is there somewhat coarser in its striae. 



Formation and locality. Hot rare in the upper part of the Trenton shales at Minneapolis, St. Paul, 

 Cannon Falls, Rochester and Fountain, Minnesota. Bare near the top of the Birdseye limestone two 

 miles south of High Bridge, Kentucky. Common in the "Glade limestone" at Lebanon and elsewhere 

 in middle Tennessee. 



Collectors. W. H. Soofleld, E. O. Ulrich and the writers. 

 Mus. Keg. Nos. 279, 346, 3515, 4049, 4935, 5149, 6778, 6804. 



ORTHIS (DALMANELLA) AMCENA N. H. Winchell. 



PLATE XXXIII, FIGS. 48 and 49. 



1880. Orthis amcena N. H. WINCHELL. Eighth Annual Report of the Geological and Natural 



History Survey of Minnesota, p. 65. 



Original description: "Shell transversely oval with a hinge-line that compares 

 to the greatest diameter about as five to nine. Evenly rounded from the cardinal 

 extremities, which hardly disturb the symmetry of the outline, through the front 

 margin; valves nearly equal; umbonal region of the receiving [ventral] valve sur- 

 rounded by a depressed or somewhat concave border, which in the front margin 

 becomes flat or inclines toward the entering [dorsal] valve; the entering valve having 

 a much less marginal concavity, but being moderately and evenly convex; cardinal 

 areas small; foramen [delthyrium] also small; beak of the receiving valve somewhat 

 incurved; that of the entering valve small, but abrupt and distinct, surface marked 

 by rays which are doubled or tripled in number on the umbo by implantation, but 

 maintain a larger size than the rest in passing to the margin, several of which are 

 also curved so as to run out in the hinge-line; transverse diameter, nine to ten lines; 

 perpendicular diameter from seven and a half to eight and a half lines. Interior 

 unknown." 



Two somewhat compressed type specimens are the only ones known of this 

 species. They differ but slightly from 0. subcequata, as the striae which originate 

 on the umbo increase in prominence to the anterior margin, between which are two 

 or three smaller ones. A very similar stria tion is seen in 0. (D.) stonensis Saffbrd, a 

 species also belonging to the 0. subcequata section, but always smaller and narrower 

 than 0. amcena Winchell. 



