440 THE PALEONTOLOGY OF MINNESOTA. 
[Ortllis (Dalniiinella) hainburscusis. 
Orthis (Dalmanella) hamburgensis? Walcott. 
PLATE XXXIII. FIGS. 14-16 
1884. Orihis hamburgensis Walcott. Monograph of the U. S. Geological Survey, vol. viii, p. 73, 
pi. II, fig. 5. 
Original description: " Shell small, suborbicular in outline, plano-convex; hinge- 
line a little shorter than the greatest breadth of the valves. Dorsal [ventral] valve 
moderately convex, most elevated a little behind the center, along a slight ridge 
formed by two or three strong, slightly raised strise; beak small, depressed about 
halfvray down the cardinal margin. Ventral [dorsal] valve depressed, the slightly 
convex, mesial depression well defined from the beak to the front margin. 
" Surface of both valves marked by from twelve to eighteen strong angular striae, 
which increase by bifurcation or intercalation towards the margin." 
The following description is based on Minnesota material : Shell very small, 
subquadrate in outline, hinge-line equal to, or slightly less than, the greatest width 
below; cardinal angles rectangular; sides in the posterior third straight or gently 
convex; anterior angles and front broadly rounded. Ventral valve convex, subcar- 
inate along the middle, with a flat slope toward the sides; greatest elevation about 
one-third the length of the shell from the posterior margin. Cardinal area wide, 
broadly triangular, slightly concave, forming an angle of about 115° with the plane 
of the lateral margin; delthyrium narrowly triangular, more than twice as long as 
wide, with a linear elevation along each wall. Beak slightly incurved, raised above 
that of the dorsal valve. Dorsal valve less convex than the ventral, with a sinus 
beginning immediatly below the beak and rapidly expanding into a broad, shallow 
depression, which produces a more or less undulated anterior margin. Cardinal 
area conspicuous, slightly concave, less than half as wide and more erect than that 
of the other valve; delthyrium about as long as wide, bounded on each side by a 
linear elevation, and occupied centrally by a narrow and simple cardinal process. 
Surface with moderately strong, radiating, bifurcating, angular striae, of which 
from thirty-two to forty-six may be counted along the anterior margin. In some 
specimens the striae are nearly equal in size, while in others those originating on 
the umbones increase rapidly in strength, and the ones coming in later by bifur- 
cation remain smaller, giving to such shells the appearance of strige in bundles. 
Interior features unknown, except that the dental plates are strong and attached 
to the bottom of the valve. 
Since the cardinal areas, delthyrium and interior characters are unknown in the 
Nevada specimens, the writers are not satisfied that the Trenton forms are identical 
with 0. hamburgensis. A specimen of the latter was sent to Mr. Walcott, who writes 
that "the cardinal line is a trifle shorter in the Nevada specimens than in those from 
