THE MOLLUSC A OF MUSCATINE COUNTY. 13 



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STREPOMATID^J. 



PLEUROCERA. 



Pleurocera subulare, LEA. Was very abundant at the mouth of 

 Burdett's Slough three }'ears ago, but we have hardly found it 

 since. "Dead" shells abound along the Cedar. Length of 

 shell, 23 ; of aperture, 7 ; diam., 7 mm. 



GONIOBASIS. 



G. cubicoides, ANTHONY. A few " dead " shells have been found 

 on Cedar River. Length of shell, 16.6; of aperture, 7; 

 diam., 8.5 mm. 



UNIONID^. 



UNIO. 



IT. sesopus, GREEN. Common. Mississippi; at Muscatine. Thick 

 near umbones; heavy; epidermis in young shells light yellow, 

 dark in old individuals. Length, 8.5; breadth, 6; diam., 

 4.5 cm. 



U. alatus, SAY. Common in Mississippi. Old shells apparent!} 7 

 nons} T mphanote ; nacre purple. Length, 15 ; breadth, 9 ; 

 diam., 5 cm. 



U. anodontoides, LEA. Abundant in Mississippi ; found also in 

 the Cedar. There seem to be two forms of this fine shell. 

 Form a seems to prefer sand ; epidermis pale yellow ; no rays ; 

 umbones plain ; is much larger than b: length, 12.8 ; breadth, 

 6.5 ; diam., 5.6 cm. Form b prefers muddy sloughs, is beau- 

 tifully rayed with green, and umbones are undulated : length, 

 8 ; breadth, 3.5 ; diam., 3.2 cm. 



U. asperrimus, LEA. Somewhat rare. Mississippi River. Two 

 forms abound here. One agrees with figure and description in 

 Lea's "Observations on the Genus Unio," except the shell 

 is much narrower posteriorly, the umbones are considerably 

 more elevated, and somewhat more anterior, than in the figure, 

 Plate V., vol. iv. : length, 10.2 ; breadth, 7.2 ; diam., 5.3 cm. 

 The other is the same as the example figured and described in 

 Say's "American Conchology : " length, 8.7; breadth, 7.3; 

 diam., 5.3 cm. It ma}' be, the long thin form is sterile, and 

 the other is prolific. 



