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. ra7«(/a, 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. J 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, Plartt, = 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 bj' 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,|l 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. 



I PaliEOzoic Fossils, vol. i, p. 68. 1862. 



§ Bulletin No. 30, U. S. Geological Survey, p. 111. 



II PI. X, fig. 1 a. 



^ Bulletin No. 30, U. S. Geolog-ical Survey, j)l. xi, fig. 1 b. 



