20 THE MOLLUSC A OF MUSCAT1NE COUNTY. 



to he well marked. This is leptodon of Conrad. Length, 

 7.3 ; breadth, 3.7 ; diam., 1.7 cm. 



U. triangularis, BARNES. Rare. Mississippi. Prolific and ster- 

 ile forms quite distinct. Length, 4; breadth, 2.8; diam., 

 2.5 cm. 



U. trigonus, LEA. Abundant. Mississippi. Young shells light 

 yellowish brown, sometimes slightly rayed with green. Old 

 shells dark or black, and much produced posteriorly. Length, 

 7.3 ; breadth, 6.3 ; diam., 5 cm. 



U. tuberculatus, BARNES. Common. Mississippi, Cedar. Largest 

 shells from Cedar. Weight, 15.5 oz. ; length, 15 ; breadth, 

 9.1 ; diam., 6 cm. 



U. undulatus, BARNES. Rare. Mud Creek at Wilton ; common 

 at Des Moines. (See plicatus.) Length, 12 ; breadth, 8.5 ; 

 diam., 5.6 cm. 



U, ventricosus, BARNES. Mississippi, Cedar, Des Moines; com- 

 mon in the two latter streams. (See occidens.) Old shells are 

 almost destitute of rays, and substance of shell thick. Weight 

 .of specimen from Cedar, 14 oz. ; length, 14.8 ; breadth, 10.7 ; 

 diam., 7.8 cm. 



IT. wardii, LEA. Very rare in Mississippi ; common in Cedar 

 and Des Moines. A very beautiful shell. We believe we 

 can show that this species and metanevrus are the same. 

 Where metanevrus lives constantly in sand, the shells are lighter 

 in color and substance, umbones and tubercles are less promi- 

 nent, and diameter of shell is less. We have a very fine 

 specimen from the Cedar. Weight, 7.5 oz. ; length, 10.5 ; 

 breadth, 8.4; diam., 5 cm. 



U. zigzag, LEA. Common. Mississippi. After considering Lea's 

 figures and descriptions of this and donaciformis, and com- 

 paring them from different localities, we think one must at 

 least be a variet}^ of the other. (See donaciformis.) 



MARGARITANA. 



M, complanata, BARNES. Common. Mississippi, Mud Creek, 

 Des Moines. We have two forms, it may be, two species. 

 We submitted specimens to Dr. James Lewis, and he says, 

 " I think they are the same, though most persons would call 

 them distinct species : the variation in the shell is due to the 



