WALKER ON ORIGIN AND DISTRIBUTION OF MOLLUSCA. 59 



and Devonian strata of Europe and America have been referred to the 

 Unionidce by different authors, the accuracy of such reference has been 

 seriously questioned and American palaeontologists have not generally 

 recognized as belonging to that family any shells found in the strata 

 earlier than Mesozoic time. "Beginning with the Jurassic period, how- 

 ever, undoubted Unionidce are found and toward the end of the Creta- 

 ceous age a large and greatly differentiated fauna is found in both the 

 new and old world." "As a rule the types that have hitherto been dis- 

 covered in the Mesozoic and Cenozoic strata of the western part of North 

 America, are such as now exist in different parts of the continent, espec- 

 ially 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 the Mesozoic epochs * * * and 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 species is such as to leave no doubt that the former represent the 

 latter ancestrally. The fact also that the types of these Mollusks had 

 become so differentiated before the close of Mesozoic time, and that they 

 have changed so little since, points back to a previous evolutionary 

 history, which doubtlessly began in Paheozoic time." 



The separation of Mesozoic sea from the open ocean and its gradual 

 change of its waters from salt to fresh, was undoubtedly the cause of the 

 great diversity of type, which so early developed itself in this family 

 during that epoch, and which has, in this country, been perpetuated to the 

 present time. "It is well known that the maximum of differentiation 

 of mulluscan types takes place in marine waters, that it is much less in 

 brackish waters, and that the minimum in this respect is reached in 

 purely fresh waters." "We should, therefore, naturally expect to find 

 in those strata which bear evidence of having been deposited in purely 

 fresh waters a fauna meagre both in species and development, while in 

 those strata that have evidence of having been deposited in waters which 

 were a little salt, the Unionidce would be much more differentiated." 

 "This is exactly what is found to be the case, and indeed it is only in the 

 last mentioned strata alone, that those species of Unio have been found, 

 that possess the peculiar North American characteristics." "Judging 

 from these facts, it would seem that these' ancient Unionidce were not 

 only capable of living in waters that were a little salt, but that the in- 

 fluence of the salt upon them was such as is in a general way exerted 

 by it upon all molluscan life producing a greater differentiation than 

 would have been produced in fresh lacustrine waters, and such as has 

 generally been supposed to have exerted upon the family in existing 

 fluviatile waters. While it is not unreasonable to assume, that much 

 of the differentation that now prevails in the living North American 

 Unionidce took place in fresh waters, the facts brought out by the study 

 of the fossil forms seem to indicate plainly, that the characteristics which 

 we call "North American" have been directly inherited from these fossil 

 species, and the possibility also, that the later species received in Mesozoic 

 and Tertiary times, their -differentation under the influences of other 

 conditions, among which was the diffusion of a small portion of salt in 

 the waters in which they lived." 



"If it be assumed, therefore, as is believed to be the case that the con- 



