26 IOWA ACADEMY OF SCIENCES. 



forms and the wide geographic I'ange of many of the varieties has perhaps 

 no parallel elsewhere. The first of these statements requires no further 

 procf to one who has worked anywhere within the limits of the region 

 under consideration. The second proposition finds ample evidence in col- 

 lections of more than sixty diffei*ent kinds of these moUusks from a single 

 locality. Altogether more than seven hundred species of the family Union- 

 idfe have been described from North America— over four-fifths of the entire 

 number known to exist in the world. Having such a large number of 

 closely related forms to deal with, it has become very convenient, and 

 indeed very necessary, to separate the chief genus into a number of sub- 

 ordinate groups, naming each after its leading species; thus the sections 

 are known as the "gibbosus," "undulatus" groups, etc. 



The distribution in space of the uniones of the continental interior has 

 been shown to be in many respects very peculiar. As the problem finds no 

 satisfactory solution in an ordinary zoological treatment, an inquiry has nat- 

 urally been made in regard to how far the present regional disposition of 

 the various groups may have been determined by the conditions of former 

 geological epochs. This involves by far the most important factor in the 

 consideration of the present geographic distribution of organisms, and one 

 which continually assumes gi'eater and greater prominence in dealing with 

 facts pertaining to that subject. 



It has also been clearly shown in other zoological families that the range 

 of many genera and species in time is very much more extended than has 

 been generally regarded, and that some of the living types have a high 

 antiquity. The recent discoveries of rich land and freshwater fauna; in the 

 Mesozoic and later deposits of the Northwest have done much toward elucid- 

 ating the early history of American fluviatile mollusks. White* has already 

 intimated in a general way the probable close genetic relationship of these 

 fossil uniones and the forms now living in the waters tributary to the Missis- 

 sippi river, but no specific references were made to the mollusks now exist- 

 ing. Laterf it was incidentally mentioned that among the Laramie Unionidse 

 were found the prototypes of Unio Ugamentiims, U. unchilatus and other 

 groups. 



In the upper Mississippi region the Unionida; are easily separable into 

 three grand sections which are commonly ranked as genera: Anodonta, 

 Margaritaua and Unio. The generic distinctions are based entirely upon 

 the characters of the hinge "teeth;" but there are also other good structural 

 features to support this separation; and the ti'ansitions are few and not well 

 marked. The leading North American groups of Unio may be typified by 

 the following species: Uaio ligamentinus Lam., U. undulatus Barnes, U. 

 ellipsis Lea, U. gibbosus Barnes. U. tuberculatus Barnes, U. pustulosus Lea 

 and U. parvus Lea, besides others which have no bearing in the present con- 

 nection. Of these at least five groups are known to have fossil representations 

 in some portion ot the western Cretaceous or Tertiary strata. In the pi'es- 

 ent consideration no forms from rocks earlier than the Mesozoic age are con- 

 sidered, for the reason that so much doubt at present exists concerning the 

 shells referred to the Unioaidaj from the Davoaian and Carboniferous of 

 this country. As regards the Tertiary forms described under Anodonta and 



*U. S. Geol. Sur.. 3rd Ann. Bep., 1883. 



tKeyes: Annot. Cat. Iowa Moi., Bui. Essex Inst,, vol. xx, 



