546 THE PALEONTOLOGY OF MINNffiOTA. ^oyrtodonta 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 TJlrich, is a larger and- 

 almost circular shell. 



Although the hinge and internal characters are unknown, I cannot doubt that 

 the species is a~true Cyrtodonta. I 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 TJlrich. 



PLATE XL. FIGS. 15-19. 



Cyprieardites tenellus Uleich, 1892. Nineteenth Ann. Kep. Geol. and Nat. Hist. Sur. Minn., p. 237. 



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 striae, 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. dictcea 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- 

 veniral 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 Eiver 

 group, are very similar shells, diflfering chiefly in the greater strength of their hinges. 



