28 IOWA ACADEMY OF SCIENCES. 



Among the fossils already alluded to are a number of Anodontte, the 

 most prominent of which is A. prepatoris, a member of the "grandis" group. 

 Margaritana has been reported from the Cretaceous, but at present there is 

 much doubt as to the correct reference of the form to this genus. 



At present the oldest American form of Unio is U. cristonensis Meek. It 

 was described from a horizon doubtfully referable to the Triassic, and was 

 first figured by White. The type spec'mens are imperfect, but show dis- 

 tinctly the generic characters. 



White* has expressly called attention to the fact of the extreme shorten, 

 ing of the Laramie Uniones in front of the beaks, or rather the forward posi- 

 tion of the umbones as compared with the modern shells. This fact is of 

 great interest in its bearing upon the phylogeny of the group, as it is au 

 important consideration in support of Neumayr's recently advanced sugges- 

 tion concerning the derivation of the Uniones from the Trigonidfe. Should 

 this near relationship of the two families be established it is very probable 

 that the view just mentioned would require some slight modification. For 

 the two families had ah'eady, in the Cretaceous, become very much diffei'- 

 entiated, so that the two types were probably derived from a common, but 

 rather remote ancestor, rather than one fi'om the other. 



A most remarkable feature concerning the Unione fauna of North 

 America is the striking individuality of the forms of each drainage basin, 

 however limited it may be. This peculiarity is so marked that one 

 acquainted with the American species of the family has little difficulty in 

 telling from which particular portion of the country, or indeed the stream, 

 a given series of shells was taken, even when the most widely distributed 

 species are under consideration. It was probably this fact more than any 

 other that occasioned the vast multiplication of species by Lea whose wide 

 familiarity with these bivalves enabled him from the external characters 

 alone to readily determine tlie locality of the various forms of Unionidaa 

 brought to his notice. It is, perhaps, one of the best known examples show- 

 ing how persistent, how exclusive, how united a particular fauna of a lim- 

 ited geographic area may be, when the physical conditions are seemingly 

 quite diverse. It also illustrates how well the peculiarities of two contigu- 

 ous basins may be fully preserved even when the conditions of environment 

 are presumably the same. A hint towards a partial explanation of these 

 phenomena is derived from geological data concerning the permanency of 

 river basins; for it has been satisfactorily shown that the water courses are 

 among the longest lived of all the topographical features of a region. This 

 being the case the Unionida3 would be admirably adapted to flourish through 

 long periods of time and undergo but slight structural modifications, and 

 this certainly seems true of these bivalves in the Missouri basin, for they 

 have come down from Mesozoic tiiues almost unchanged. 



The distinctness of unione faunte in separate drainage basins has some 

 striking illustrations in the upper Mississippi valley. One in particular 

 has I'ecently been brought into notice in the case of the Des Moines and 

 Iowa rivers, which flow parallel to one another southeastward across the 

 State of Iowa. The peculiar distribution of the lamellibranchs in the 

 eastern and western portions of the State was pointed out some time ago in 

 an annotated catalogue of the Mollusca of Iowa. Of the species found in 



*U. S. Geol. Sur., 3rd Ann. Rep., p. 431, 



