The OaleiKi and Jr<((juo/iefa Series. — S((rdeson. 105 
ture specimens of that species. The breadth of the shells is about one- 
fourth greater than the length. The hinge is a little shorter than the 
whole width of the shell, and the cardinal angles are rounded. The 
ventral valve has the beak but little curved, and projecting over the car- 
dinal area but little. Its convexity is small, and the median elevation, 
which is sometimes strong, produces a depression on either side of it. 
The dorsal valve has a median depression, with convex areas on either 
side. 
In these characters great similarity to O. eiiiacerata is shown. The 
internal characters, seen only in two immature specimens, show also 
close relation to O. einacerata, to which this one might be referred as a 
variety, if their relation as such could be traced. 
General Discussion. 
Of the above described Ort/u's, one only is from the Galena 
series; the others are found in the Maqiioketa reries or its 
<'({uivalent, the Cincinnati series of Indiana and Ohio. The 
Trenton species, O. rofjata Sardeson, is the geologic predeces- 
sor of them all in the area wherein they are found, and ma}^ 
be the one from which some of them were derived. Morplio- 
logically the large individuals of 0. rogafa Sar. are the ones 
that most resemble O. einacerata Hall, by which O. porrecta 
n. sp. and O.fiif/lis n. sp. are approached in nearly direct 
lines. 0. royata Sardeson approaches 0. corpulenta Sar. in 
many characters and particularly in the extent and direction 
of variations, but the latter is very much the larger, while re- 
sembling most the dwarfish specimens of the former. Dwarfed 
specimens of 0. rogata Sar. show characters not seen in large 
ones, but whicli are normal in its successor 0. niulfisecta Meek 
and this species has the same characters advanced as com- 
pared to the dwarfed specimens of O. rogafa., but less ad- 
vanced than the same in O. tersa Sar. O. nieeki S. A. Miller 
and O. ignota n. sp. are in other directions advanced from the 
<levelopment as seen in (). mulfisecfa Meek. A few specimens 
from the typical Trenton of New York, whicli have been ex- 
amined, are of two kinds, and are easily referable to Orthis 
cviacerata Hall and O. corpulenta Sardeson, respectively, as 
their Trenton lepresentatives. The two Trenton species are 
l)ractically, if not exactly, contemporaneous with the one 
Galena species. The three of the Trenton period represent 
those of the Hudson period, so far as I now know, as follows: 
(K emacerata. var,, in the Trenton is followed by the typical 
O. emacerata Hall, 0. porrecta n. sp., and (). fntilis n. sj)., in 
