Pei cen; LAMELLIBRANCHIATA. 571 
small, rounded, situated in the antero-dorsal angle. Posterior scar and pallial line 
undetermined. 
In most specimens there are two or three, thin, parallel and oblique ridges on 
each side at the extremity of the hinge. These opposed sets of ridges are separated 
by an interval in adult examples, but it is scarcely to be questioned that in an earlier 
stage in the development of the shell they represented posterior lateral teeth similar 
to those of Cyrtodonta. This fact must have an important bearing upon the question 
of genealogy, but, in the absence of any knowledge of similar types in earlier strata, 
it is not now possible to discuss it with anything like certainty of arriving at a true 
solution of the question. 
This is a well*marked species and one that is not likely to be confounded with 
any of the associated shells. Its nearest congeners seem to be W. scofieldi and W. 
sterlingensis M. and W., sp., the latter particularly, but in both of these species the 
umbonal ridge is more conspicuous and the outline different. The former again has 
a longer escutcheon and hinge, much larger anterior end and more prominent 
postero-cardinal angle, while in the latter the shell is more oblique, the posterior 
angle narrower, the cardinal area much wider and the beaks farther apart. 
Formation and locality.—Lower Trenton limestone, near Beloit, Wisconsin and Minneapolis, Minn. 
Wairetnia scorietpi Ulrich. 
PLATE XULJ, FIGS. 17—21. 
Whitella scofieldi ULRICH, 1890. Amer. Geol., vol. vi. pp. 181 and 382. 
Shell of medium size, strongly convex, moderately oblique, subtrapezoidal in 
outline, with the hinge line longer, straighter and better defined than usual. Ante- 
rior end unusually long and wide, the outline gently rounded from the subangular 
junction with the hinge line; basal margin slightly convex, oblique, descending to the 
strongly rounded postero-basal angle; posterior margin subtruncate, slightly oblique 
and but little convex in the upper half. Umbones prominent, subcarinate behind, with 
the beaks approximate, obliquely enrolled and situated a little more than one-third of 
the length of the hinge line behind its anterior extremity. The umbonal ridge is a 
conspicious feature, although becoming obsolete before reaching the postero-basal 
margin. Posterior to the ridge the surface is distinctly concave; in front and 
beneath it convex. Surface marked by rather distinct concentric lines of growth, 
of which the marginal ones may, in old examples, assume a sublamellose character. 
Escutcheon high but narrow in a dorsal view, finely striated longitudinally and not 
extending anterior to the beaks. Internal ligamental supports appearing as a double 
ridge in each valve beneath the posterior half of the escutcheon. Anterior hinge 
