70 
PALEONTOLOGY OF NEW YORK. 
species as identical with 0. polita, Hall * * * § but specimens of both species, from the 
original localities of each, show differences in the much less convexity of 0. 
nana, its more triangular outline, and its usually smaller size. Should these 
differences prove constant, the species may be regarded as well defined. 
0. nitida, Ford.f Mr. Ford’s specimens show a very tenuous phosphatic shell 
which has not retained any trace of the internal characters. 
0. pretiosa, Billings.]: Interiors of the type unknown. Mr. Walcott suggests 
their relationship to Acrothele ;§ but specimens which have been placed in our 
hands by Sir William Dawson, from the Quebec group at Little Metis, and 
which show no external differences from 0. pretiosa as described by Billings, 
have all the internal characters of the genus Linnarssonia. 
0. transversa , Hartt, = Linnarssonia, Walcott. 
Our discussion of the generic characters of Obolella must, therefore, be 
limited to observations made upon the authentic species cited. 
Primarily, a distinct cardinal area is developed in each valve. On the 
pedicle-valve this feature is much the more conspicuous, and is crossed by a 
pedicle-groove, which is not a slit cut through the area, but only a depression 
on its surface. The brachial valve on the other hand shows only a broad 
sinuate depression as seen in the figure of 0. gemma given on Plate II (fig. 43 
[34 in error]); in 0. crassa this valve has a somewhat triangular area with a 
very slight ridge occupying a position correlative to the pedicle-groove of the 
other valve. Mr. Ford’s figure, given in Bulletin No. 30, United States 
Geological Survey,|| makes this feature much stronger than it actually appears 
in the specimen from which the drawing was made. Though the same feature 
is shown in Mr. Walcott’s figure of the brachial valve of 0. chromatica it 
remains to be determined whether or not it is a constant character in these 
species. 
* Bulletin No. 30, U. S. Geological Survey, p. 111. 
t American Journal of Science, 1873, vol. v, p. 213. 
J Palaeozoic Fossils, vol. i, p. 68. 1862. 
§ Bulletin No. 30, U. S. Geological Survey, p. 111. 
| PL x, fig. 1 a. 
If Bulletin No. 30, U. S. Geological Survey, pi. xi, fig. 1 b. 
