Sterki : Some Notes on Sph^riid-e. 439 



greatly prominent in the full-grown; the sulci are somewhat coarse 

 and rather irregular to slight and almost obsolete ; the color is lighter 

 or darker grayish corneous. The hinge is somewhat more curved, 

 the posterior laminae being at a noticeable angle to the plate. It 

 should be added that mussels taken for aurcum are regularly and 

 evenly rounded-inflated, while in dcclive the disks — i. e., the middle 

 part of the valves — are somewhat flattened. Long. 14.5; alt. 11.5; 

 diam. 8.8 mm. 



Typical specimens, well representing the species, are C. M., No. 

 9140, from the Sheyenne River, near Tolina, North Dakota, collected 

 by Mina L. Winslow, July 26, 1919; other specimens of the same lot 

 are in the collection of the Museum of Zoology, University of Mich- 

 igan. There are also numerous specimens from Deuel County, South 

 Dakota, collected by W. H. Over in 1896; Bates Creek, C. M. Nos. 

 5212, 5315, etc.; and a small creek near Clear Lake, C. M. No. 5133, 

 etc.; the former are larger, with heavier shells and hinges; long. 16.4; 

 alt. 12.5; diam. 10.5 mm. 



