IOWA ACADEMY OF SCIENCES. 27 



Margaritana considerable confusion also prevails; and it is quite certain that 

 some of the species have been wrongly referred to these genera. 



It has been' stated by Binney and others that among the living land inol- 

 lusks a wide geographic distribution is indicative of a high antiquity for the 

 group. This observation has lately* been extended to certain Carboniferous 

 moUusks. By carefully reviewing the American Unionidai it will be found 

 that the generalization is applicable to this family also. Those (subgeneric) 

 groups having the widest geographical range at the present time in the 

 basin of the Mississippi river are the ones which are best i-epresonted in the 

 Mesozoic strata of the upper Missouri region. As examples. Unio ligamen- 

 tinus, U. ellipsis, U. undnlalus and U. rectus are the most prominent, per- 

 haps. These four species range from Ottawa, Canada, and western New 

 York, to southwestern Kansas and Texas, and from Alabama to northern 

 Minnesota and Dakota. All four groups, along with others, are present in 

 considerable numbers in the freshwater Lai'amie deposits of the Northwest. 



Of the group typified by the first species mentioned — Unio Ugamentinus 

 — there are a number of forms now known among the fossil Uniones. The 

 shell of the living representative is exceedingly variable, as might naturally 

 be expected of a species occurring under the many diverse conditions of 

 environment such as are imposed by its wide geographical distribution. 

 Throughout its range many specific terms have been applied to the various 

 varietal forms. In some localities this species has a very thin and fragile 

 shell; in others the shell is very thick and massive, with large, heavy hinge- 

 teeth, and rough, deep, muscular impressions, resembling in many respects 

 the early described Unio crassidens of Lamarck. To the latter category the 

 majority of the Laramie forms of the group appear most closely to aijproach, 

 particularly such shells as Unio velustus Meek, from southwestern Wyoming. 

 U. prisons M. and H., seems also to belong to the group. The type contin- 

 ued through the Eocene as U. shoshonensis White. 



Unio ellipsis is the type of a rather large and variable group of shells. 

 The beaks in this species are far forward, even extending beyond the anterior 

 margin of the shell. It is thus a representative of a series having but few 

 examples among the forms at present living, but which was almost universal 

 among the Laramie species, as was first pointed out by W^hite. The most 

 nearly related of the fossil species now known is perhaps U. proavilus W., 

 but in the former the "teeth" are somewhat heavier and the outline more 

 rotund. Other forms of this type are are found in V. cryptorhynchus and 

 U. propheticus. 



Unio gonio7iotis White, is evidently one of the "undulatus" group; but 

 it more closely resembles some other members of this section rather than 

 the leading species itself, U. belliplicatus, while differing considerably from 

 the type of the group, is believed to have a close relationship with other 

 members of the section, particularly certain forms that have recently been 

 noted from Kansas. 



The Unio recttts gi'oup is characterized by rather large, elongate forms, 

 having heavy shells, rounded in front, and more or less attenuated behind. 

 The Laramie representatives are best known under U. co7iesiW., and per- 

 haps also U. daiia; ^l. and H. In the Eocene U. clinopisthus appears to 

 have flourished as the; descendant of the early "gibbosus," or "rectus" type- 



•Proc. Acad. Nat. Sci., Phlla., 1888, p. 245. 



