ART. XXIII.-PALEONTOLOGICAL PAPERS NO. 4. -COMPARI- 

 SON OF THE NORTH AMERICAN MESOZOIC AND CENOZOIC 

 UNIONID.E AND ASSOCIATED MOLLUSKS WITH LIVING 

 SPECIES. 



By C. a. White, M. D. 



The remarks euibraced iu this paper are intended to apply only to 

 those fossils which are enumerated iu the catalogue that constitutes the 

 principal part of the paper immediately preceding this (Paleoutological 

 Papers Xo. 3), and also to that portion of North America to which the 

 catalogue is restricted. 



The scope of this paper is still further restricted by treating only of 

 those MoUuscan forms that are universally regarded as indicating either 

 a purely fresh-water or a land habitat. Furthermore, an extended dis- 

 cussion of the relations and characteristics of the Gasteropoda of that 

 catalogue is deferred until another occasion shall offer, the present one 

 being dervoted mainly to remarks upon the fossil Unionidce, and their 

 relation to living forms of the same family. 



As a rule, the types of fresh- water and landMollusks that have hitherto 

 been discovered in the Mesozoic and Cenozoic strata of the western part 

 of Xorth ximerica are such as now exist in different parts of the conti- 

 nent, especially its eastern half. This similarity of type, although it is 

 somewhat more apparent in the later than in the earlier formations, 

 extends as far back as thfe earlier Mesozoic epochs, and is so plainly 

 apparent that the principal indication of antiquity which the specimens 

 exhibit is their fossilized condition. Even in the case of a majority 

 of exceptions to this rule, the relationship to existing forms is readily 

 recognized. 



In short, the almost exact identity of types of the fossil and living 

 fresh-water and land Mollusca of Xorth America is such as to leave no 

 doubt that the former represent the latter ancestrally. The fact also that 

 the types of these fresh-water and land MoUusks had become so variously 

 differentiated before the close of Mesozoic time, and that they have changed 

 so little since, points back to a previous evolutional history which doubt- 

 less began in Paleozoic time, but concerning which we have yet col- 

 lected little or no material. So large a proportion of the existing genera 

 and subgenera of North American fresh-water and land Mollusks have 

 been found to be represented by fossil extinct species, that it is probable 



