546 THE PALEONTOLOGY OF MINNESOTA. 
[Cyrtodonta tenella. 
This species seems to be rather closely related to Cyrtodonta canadensis Billings, 
but is more erect, comparatively higher posteriorly and has its outline more pro- 
duced and more sharply rounded in the postero-cardinal region. C. tenella has a 
longer hinge line and is more uniformly convex. C. grandis Ulrich, is a,larger and 
almost circular shell. 3 
Although the hinge and internal characters are unknown, I cannot doubt that 
the species isa true Cyrtodonta. [judge further that it belongs to the group of species 
of which C. persimilis and C. rotulata are typical members, 
Formation and locality.—Middle third of the Trenton shales, Minneapolis, Minnesota. 
CyrTODONTA TENELLA Ulyich. 
PLATE XL. FIGS. 15-19. 
Cypricardites tenellus ULRICH, 1892. Nineteenth Ann. Rep. Geol. and Nat. Hist. Sur. Minn., p. 287. 
Shell of medium size or less, moderately ventricose, not very oblique, subovate, 
widest posteriorly, slightly alate and subangular or sharply rounded in the postero- 
cardinal region. Hinge line long, slightly arcuate, posterior margin straightened 
in the upper half, broadly rounded and produced a little in the lower half; ventral 
margin rather strongly convex, most prominent a little behind the middle; anterior 
end more or less narrowly rounded. Beaks small, incurved, projecting moderately 
beyond the hinge line; situated about one-fourth of the entire length behind the 
anterior extremity; umbones full, prominently rounded. Cardinal slope slightly 
concave. Surface marked with rather fine concentric striz, and sometimes with 
strong distant lines of growth as well. 
Shell substance very thin. Hinge plate narrow, a good part of it forming a finely 
striated ligamental area; two very slender posterior lateral teeth in the right valve, 
and the same number probably in the left; anterior teeth obscure in the specimen, 
consisting apparently of two slight horizontal folds in the margin of the shell, 
muscular impressions very faint. 
The hinge plate and teeth are thinner in this species than in any other known 
from Lower Silurian deposits. In two Upper Silurian species, however, C. primigenia 
Conrad, sp. (Medina), and C, dictwa Hall, sp. (Niagara), the hinge is quite as slender 
if not more so. 
C. cingulata is a more ventricose shell, especially in the central and umbonal 
regions; the outline is a little different, being longer from the beaks to the postero- 
ventral margin, and the hinge line shorter. C. grandis and its varieties germana and 
luculenta, the first and second from the Galena, the last from the Hudson River 
group, are very similar shells, differing chiefly in the greater strength of their hinges. 
