148 PALEOZOIC PALEONTOLOGY. 
CRANIA sp. undet. ; 
Plate IX", Wig. 9: 
A few imperfect specimens of a species of Crania have been ob- 
served in the Trenton limestone of New Jersey. They resemble C. 
trentonensis Hall, but the apex of the brachial valve is usually close 
to the posterior margin. 
RAFINESQUINA ALTERNATA (Emm.). 
Plate IX., Figs. 12-13. 
1873. Strophomena alternata Meek, Pal. Ohio, vol. I., p. 88, pl. 7, 
Re Mics 
figs. 1-3. 
Description.—Shell semi-elliptical or semi-circular in outline; great- 
est breadth of the shell along the hinge-line; cardinal extremities 
rectangular or sometimes more acutely angular, sometimes compressed 
and moderately deflected; lateral margins a little convex or slightly 
sinuous posteriorly, rounding forward to the front, which is semi- 
circular in outline. Pedicle valve depressed, convex at the umbo, 
but usually much compressed over the whole visceral region in the 
adult (which includes the whole surface of the young and half-grown 
shells), becoming more convex, frequently more or less geniculate,, 
anteriorly and laterally, and thence more or less curved to the mar- 
gins of the shell. Cardinal area of moderate height, flat and directed 
obliquely backward, nearly at right angles to that of the other valve ; 
beak small, scarcely distinct from the margin of the area, minutely 
perforated; delthvrium broadly triangular and arched over by the 
deltidium, which is deeply sinuous on its inner edge, the sinus being 
nearly or quite closed by the cardinal process and chilidium of the 
other valve. 
Brachial valve flattened on the umbonal and cardinal regions, 
gently or more or less strongly concave in the central and anterior 
portions, and curved upward around the anterior and lateral margins ; 
beak small, but projecting slightly beyond the edge of the area, which 
is very narrow or sublinear, and directed nearly in the plane of the 
wv 
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