174 witter: unio luteolus and its allied forms. 



Considerable confusion exists concerning this species, It seems 

 to me that we have tliree forms near Muscatine known as luteohis; 

 of these three forms I should make two groups; in the first group 

 there might be placed those forms with numerous delicate undu- 

 lations or folds about the beaks, and the shells are usually a little 

 less than twice as long as wide, there is a large percentage of green 

 in the color of the epidermis and some examples are very beautiful; 

 this group seems with us to prefer small and rather sluggish creeks 

 or sloughs with muddy bottoms. I shall designate the first group 

 as (A), and the second as ( B ). (A), is capable of subdivision 

 and these forms may be representsd by (a), and (/3). (a), is much 

 smaller and lighter in every way than (/3), umbones very low, and 

 generally less brilliantly colored. I have not found the two forms 

 (a) and (5) in the same waters. The largest of the form (a), mea- 

 sures as follows: length 7.8, breadth 4^4, diameter 2.2 cm. 



(/3), has rather prominent umbones, is darker in color and a 

 larger and heavier shell, the undulations about the beaks in these 

 two forms appears to be the same. (^) measures as follows, length 

 1 1.3, breadth 6, diameter 4.5 cm. There seems to be but little 

 difference in t!,e size of mature shells, except the diameter. In 

 this respect there is a very marked exception in both forms. A 

 thin form, esiDecially flattened and somewhat pointed posteriorly, 

 and a tumid form, truncated posteriorly. These [ have regarded 

 as fertile and sterile respectively. Group (^^5^, contains forms in 

 most respects very different from those mentioned above. The 

 epidermis is often yellow, with scarcely a line of green to be seen, 

 and is highly polislied. The umbones are high, in some as well 

 developed as in occidens, Lea. At the beaks are two or three coarse 

 folds or wrinkles. The hinge teeth appear to be the same as those 

 q{ occidens. In tliis group there is a form somewhat pointed pos- 

 teriorly and another with a most conspicuously truncated posterior 

 and very wide behind the umbones. A large example of group 



J.C, iii., April, 1S81 



