528 THE PALEONTOLOGY OF MINNESOTA. 
{Endodesma orthonotum. 
sharply defined. In the cast the dorsal edge from the beaks to near the posterior 
extremity of the hinge appears bent inward and downward. Surface marked with 
moderately distinct concentric striz of growth, crossed on the dorsal slope by an 
obscure sulcus, extending obliquely from the posterior side of the beaks to the mid- 
dle of the obliquely subtruncated upper part of the posterior margin. 
Length about 64 mm., greatest posterior hight 22 mm., anterior hight 21 mm., 
convexity 20 mm. 
The above description is founded upon the original type of the species which is 
preserved in the Illinois State Museum. The obscuring matrix of which the authors 
of the species complain was removed without much trouble and a good cast pre- 
pared. The figures on plate 87 were drawn from this counterfeit of the type and 
and give a reliable idea of the species, which most certainly cannot be said of Meek 
and Worthen’s illustration. 
Comparing the species as it is now known with other forms of the genus Endo- 
desma we find that it is one, and the earliest, of three closely related forms which at 
first seemed scarcely distinguishable. As usual, however, with such hasty conclu- 
sions their error soon became manifest when careful comparisons were undertaken, 
so that now I may say that they are not only separable but with ease even when 
the specimens are complete. Thus the second of these species—the next 
described, E. wndosum--is distinguished from Meek and Worthen’s species by its 
irregularly undulating surface, more distinct growth lines, and uniformly rounded 
posterior margin. The third species, #. gesneri Billings’ sp., is nearer than LE. 
undosum, but as a comparison of figures 1, 2,3.and 4 on plate 37 will show, there 
is in this case even little trouble in drawing the specific lines. Meek and Wor- 
then in distinguishing FE. orthonotum from the Canadian species seem to have 
relied chiefly upon the more central position of the point of greatest convexity 
in their species, but this difference is much less in the specimen of FE. gesneri 
here illustrated.* We must therefore depend upon other differences among which 
I find one that seems to be well marked, namely, the anterior extremity of H. 
gesneri is subangular while in E. orthonotum it is almost regularly rounded. Car- 
rying our comparison to other points we find that in the latter the upper pos- 
terior edge is more truncated, the dorsal outline somewhat straighter, and the 
valves on the whole more convex and a little longer. 
I have rejected Worthen’s name rectiformis because under Endodesma the 
specific name orthonotum is not preoccupied as was the case under Modiolopsis. 
*In Billings’ figure 45 b (Paleozoic Fossils, vol. i, p. 43) this point is so far behind the center that I am constrained 
to believe the figure overdrawn or the specimen abnormal in this respect. 
