124 Cincinnati Soctety of Natural Fiistory. 
size: Length, 1.15 mm; height, 0.72 mm.; tieknesssesss 
mm. 
This well-marked species is intermediate in character between 
J. crepidiformis and /. digitata, the loop-like form of the ridge 
suggesting the former, while in the duplex character of the poste- 
rior arm it approaches the latter. 
Position and locality: Trenton shales, Minneapolis, Minn. 
PLACENTULA MARGINATA, DN. Sp. 
Tibate KEP e5 1 320510, 
Carapace small, vertically semi-ovate, very slightly oblique, with 
the dorsal margin nearly straight and about three-fourths as long 
as the valve. Cardinal extremities angular ; ends of carapace sub- 
equal, curving neatly into the but little less convex ventral edge. 
A wide and sharply-defined flange forms the free margin; within 
this a thin, abruptly-elevated, wall-like ridge, nearly parallel with 
the outer edge. Body of valve with a faint, semi-circular eleva- 
tion a little to one side of the center of the dorsal margin, enclos- ° 
ing the, in this species, undefined depression characterizing the 
genus. Around this elevation (beneath and on each side of it) the 
surface is slightly concave. 
size: Length, 0-52 mm height. oss 7mm 
This and the succeeding species differ from the typical species 
of the genus in having the loop-like elevation or ridge and enclosed 
depression of the dorsal slope less developed. It is, however, 
scarcely to be doubted that they are true species of the genus. 
The thin sub-marginal ridge and flat border of P. marginata 
are such distinctive features that the species is not likely to be 
confounded with any other known to me from the Cincinnati 
rocks. } 
Position and locality: Cincinnati Group; rare at Cincinnati, 
Ohio, where it occurs at the top of the hills. 
PLACENTULA INORNATA, 0. Sp. : 
Plate X, Figs. 14a, 0. 
This species is in many respects like the preceding, but is 
longer, has the anterior end narrower than the posterior, and is 
without the thin sub-marginal ridge and flange. » Instead, the mar- 
