THE MOLL USC A OF MUKCATINE COUNTY. 15 



IT. ebenus, LEA. Common. Mississippi River. We have shells 

 of this species of all ages ; and none of them seem to be 

 marked with yellow behind the beaks, as figured and described 

 by Lea. Length, 8.8 ; breadth, 7.5 ; diam., 5.4 cm. 



IT. elegans, LEA. Rather rare. Mississippi and Cedar. Epider- 

 mis usually rayed with green, but some are destitute of rays, 

 having a light-yellow color ; nacre generally white, but some- 

 times pink. The latter is very handsome when polished. 

 The largest shells occur in Cedar. Length, 6.7 ; breadth, 

 5.5 ; diam., 3.7 cm. 



IT. ellipsis, LEA. Common. Mississippi and Cedar. Beaks much 

 more anterior in some examples than in others ; generally 

 distinctly raj*ed. Largest shell, so far, is from the Cedar. 

 Length, 8.6 ; breadth, 6.2 ; diam., 5.2 cm. 



U, gibbosus (dilatatus, RAF.), BARNES. Mississippi. Common. 

 (See arctior.) Length, 10.5 ; breadth, 5.2 ; diam., 3.3 cm. 



IT. gracilis, BARNES. Common. Mississippi and Cedar. Seems 

 to prefer very muddy bottoms. Resembles Icevissimus (Lea) , 

 but epidermis is less polished ; generally rayed with green ; 

 posterior wing much less elevated, and no anterior wing ; 

 nacre white, with a tendency to purple back of the umbonal 

 cavity; shell very thin, and liable to crack when dry. 

 Largest shells from the Cedar. Length, 16.5; breadth, 

 10.5 ; diam., 5.2 cm. 



U. graniferus, LEA. Rare. Mississippi. This is most likely ver- 

 rucosus of Barnes, and should be so called ; is a most beautiful 

 shell when polished (the same process as for marble). Nacre 

 purple. Length, 8.5; breadth, 7.2; diam., 5.4 cm. 



U. laevissimus, LEA. Common. Mississippi and Cedar. Epi- 

 dermis more or less purple, and highly polished ; nacre 

 purple ; substance of shell very thin. Very young shells 

 are almost transparent ; and we have found them attached 

 to objects by byssal threads. Largest shell from Cedar. 

 Length, 17 ; breadth without wing, 10.5 ; diam., 4.2 cm. 



U. ligamentinus, LAM. Common. Mississippi. Epidermis in 

 3 T oung specimens green when the nacre is white ; or, when red 

 is prominent, the nacre is pink. Length, 11.3 ; breadth, 7.1 ; 

 diam., 5.2 cm. 



U, luteolus, LAM. Common in slough below Keokuk Lake. 



