Sterki: North American Sphj.riid.e. 465 



P. ovum is so characteristic that it may be recognized at once, and 

 cannot be mistaken for any other species. Forms of P. snhrotundum 

 of somewhat the same size and appearance are less inflated, less 

 oblique, having the superior margin longer and less marked-off, and 

 the hinge longer and differently formed. Its shape, and especially its 

 hinge, place P. ovum in a group with P. rotundatum, ventricosiim, etc., 

 as the largest of them. 



Habitat. — Montana, Colorado. 



It was collected by Mr. L. E. Daniels in 1912 and 1915 in the 

 Bitter Root Valley (Pacific drainage), Montana, at several localities. 

 In 1906 it was taken by Judge Junius Henderson in Boulder Creek, 

 Boulder County, Colorado, at an elevation of 9,000 ft. Specimens 

 collected by Judge Henderson were received by the writer from I\lr. 

 Bryant Walker in 1910 and shelved as " (?)." From various localities 

 in the Bitter Root Valley there come variant forms, fairly constant 

 from each place, some larger and lighter in color, others smaller and 

 darker. Two lots combined are regarded as types: specimens in the 

 collection of Mr. Daniels, and Carnegie Museum No. 7144, from the 

 eastern side of the Bitter Root River at Ward, and Carnegie Museum 

 No. 7132, from the Bitter Root River-bottom at Charlos. 



A somewhat unicjue feature found in a half-grown specimen belong- 

 ing to Carnegie Museum No. 7144 appears to be worthy of notice. 

 The right valve has two cardinal teeth. The posterior part of the 

 regular cardinal is not thicker than the rest; above it is a small, short, 

 lamellar tooth, and between the two is the groove corresponding with 

 the posterior cardinal of the left valve. In the latter there is a 

 distinct excavation bounded below by the posterior cardinal and 

 above by the nymph and the rim of the resilium for the reception of 

 the additional right cardinal. 



103. Pisidium rotundatum Prime. 



Pisidimn rotundatum Prime, Boston Proc, IV, 1851, p. 164; Mon. Corb., 1865, p. 72. 



Two poor valves apparently belonging to this species, as under- 

 stood, are contained in the T. Prime Collection. 



Habitat. — New England and New York to Ontario, Ohio, Minnesota, 

 Manitoba, Montana (Pacific drainage). It seems to occur also in 

 Europe. 



Fossil. — Michigan. 



