or THE MOLLUSCA OF IOWA. 77 



the Minnesota river about one mile above tlie railroad 

 bridge. 



Margaritana hildrethiana Lea. Des Moines river, rare ; 

 Iowa river, common at Iowa City where many were noticed 

 among the shells of other Umonidce brought out of the 

 stream by rauskrats. It is often found in large numbers 

 beneath submerged slabs of limestone which occur abun- 

 dantly along this river. 



Margaritana marginata Say. Apparently rare through- 

 out the state, but occurring in the Raccoon river in Dallas 

 county more abundantly than elsewhere. 



Margaritana monodonta Say. Eastern Iowa : not com- 

 mon. Davenport and other places on the Mississippi river. 



Margaritana rugosa Barnes. Shell Rock river ; Des 

 Moines river at Des Moines, Ottumwa and Bonaparte ; 

 Mississippi, Cedar and Iowa rivers. Quite common in the 

 Raccoon in Dallas county. 



Vnio cesopus Green. Des Moines river rather rare ; 

 eastern part of the state common. Those at Des Moines 

 are usually found in sand and have the epidermis bright 

 yellow, or straw color, while those obtained in eastern 

 Iowa are much darker, especially old specimens which are 

 very dark brown, presenting a marked contrast with the 

 bright reddish animal portions. 



Unio alatus Say. Abundant in many of the streams of 

 eastern Iowa, but not as yet reported from the central por- 

 tion of the state. This is one of the few bisymphynote 

 shells occurring in the Upper Mississippi region. It often 

 attains a large size — eight or ten inches in length — and 

 is correspondingly heavy and thick ; the young shells are 

 comparatively thin and fragile, and closely resemble JJ. 

 Icevissimus Lea. 



Unio anodontoides Lea. Abundant in eastern, rare in 

 the central, part of the state. This species has a wide dis- 



ESSEX INST. BULLETIN, VOL. XX 6 



