THE MOLLUSC A OF MUSCATINE COUNTY. 17 



IT. nmltiplicatus, LEA (heros, SAY). Very rare. Mississippi, at 

 Muscatine, and lake near Port Louisa. Length, 16 ; breadth, 

 11.2; diam., 7.2 cm. A "dead" shell found near Port 

 Louisa measures, length, 17.5 ; breadth, 12.5 cm. ; and 

 weighs 1.5 Ibs. 



IT. mississippiensis, CONRAD. Abundant in Muscatine Slough, 

 above Keokuk Lake. It abounds in nearlj* all ponds near the 

 Mississippi and Cedar. The sterile form seems to be much 

 larger than the prolific, attenuated and pointed posterior^; 

 while the latter is less flattened, and is conspicuously trun- 

 cated at the posterior extremity. Measurements of largest 

 sterile form are, length, 10.7; breadth, 4.5 ; diam., 3.4 cm. 



IT. obliquus, LAM. We have one valve from the Mississippi at 

 Muscatine, of what corresponds to shells called by this name, 

 from the Ohio. The shell is deformed, and may not be obli- 

 quus. Obliquus, as .figured and described in " Monograph of 

 the Unionidae," by Conrad, is ebenus of Lea. 



U. occidens, LEA. Very common. Mississippi and Cedar. Young 

 shells are beautiful. It varies in form, and in color of epider- 

 mis and nacre. It is often difficult to distinguish it from 

 ventricosus (Barnes). The difference cannot be due to the 

 sexes, inasmuch as thin, long forms, and tumid, abrupt, or 

 truncated forms, are found in both groups. Ventricosus is 

 generally abundant in the interior streams of this State, and 

 varies towards luteolus, whilst occidens abounds in the Mis- 

 sissippi, and runs into ventricosus. No doubt, ovatus (Say) 

 and subovatus (Lea) are included in this range of variation. 

 We have a specimen from the Cedar that answers to the 

 figure and description of subovatus in Lea's " Observations," 

 except that the umbones are not quite so far forward in our 

 shell. Ovatus (Say), figured in Conrad's "Monograph," is 

 most probably a form of our ventricosus (Say) , especially those 

 with a light-yellow epidermis, with few or no rays. It will 

 require considerable skill to make out the difference between 

 the figure of ventricosus in Say's "American Conchology," 

 and of occidens in Lea's "Observations." Length, 12.5; 

 breadth, 8.5 ; diam., 6 cm. 



II. orbiculatus, HILD. Rare. Mississippi. Shell variable. Epi- 

 dermis in young shells generally yellowish brown, and more 



