OF DENISON UNIVERSITY. 
31 
and still more faint equidistant radiating lines. These latter are not 
very numerous and could easily be overlooked. Length r.20, 
width .45. 
The radiating stri^ or impressed lines do not spring from the 
beak, but seem to terminate in the posterior hinge line ; they are only 
obvious on the posterior third of the shell. 
Genus macrodon, Lycett. 
1. Macrodon ohsoletus, Meek. 
(Plate IV, Fig. 19.) 
Shell elongate sub-quadrate or sub-rhombic, moderately convex, 
greatest convexity forming a gentle prominence extending from the 
umbo to the lower posterior margin. Upper posterior portion nearly 
plane, inclining gently to the obliquely truncated posterior margin ; 
this part of shell only marked by faint radiating striae. Hinge line 
straight, parallel to it two or three impressed lines separating the teeth, 
these showing in the partially denuded shell figured. Beaks removed 
but a short distance from the front margin. Anterior margin gently 
curved, meeting the hinge line by an abrupt right angle. Lower mar- 
gin nearly straight or slightly convex. Entire surface marked by few 
concentric lines of growth. 
2 .Macrodon tenuistriata, M. and W. 
“ Shell small, rhombic-oblong, rather distinctly convex along the 
umbonal slopes and near the front, a little more than twice as long as 
high ; basal and cardinal margins parallel, the former nearly straight 
or somewhat sinuous near the middle ; cardinal margin straight, not 
quite equalling the greatest antero-posterior diameter ; anterior side 
rounding up from below so as to meet the hinge nearly at right angles; 
posterior basal margin narrowly rounded ; posterior margin obliquely 
truncated, often a little sinuous above ; dorsal region behind the um- 
bonal slope compressed; beaks compressed, a little flattened, incurved 
and rising somewhat above the hinge margin, located about half way 
between the middle and front; flanks broadly impressed or concave 
from the umbonal region obliquely backward to the faintly sinuous 
part of the base ; surface ornamented by distinct marks of growth 
