58 MICHIGAN ACADEMY OF SCIENCE. 



as far as the Laramie period is purely a native American family and 

 ranges from the Atlantic to the Pacific and as far south as Central Am- 

 erica. Its affinities are very close to the old world family of the 

 Melaniidw, which, however, is also found in South America, but does 

 not range further north than Mexico. It is a fact of great significance 

 and one which may point to the origin of the North American family 

 that representatives of both families are found associated in the fresh 

 water deposits of the Laramie epoch. The local conditions which resulted 

 in the extinction of the Melanidw, but which permitted the survival of 

 the Pleuroceridw can only be surmised. 



The occurrence of the PleuroceHdw on the Pacific coast, where the 

 ViviparidcB are not found, is also an interesting and important circum- 

 stance. As is also the further fact that the Californian species have 

 certain peculiarities, which separate them from the eastern forms. 



While there is not at present sufficient evidence perhaps to warrant it, 

 a pleasing theory can be formulated, which accounts for the apparently 

 anomalous distribution of these families. Assuming that the Pleuroceridw 

 are an offshoot from the old world Melaniidie it would be easy to account 

 for their introduction into North America as a part of the molluscan 

 immigration in secondary times already alluded to in connection with 

 the land mollusks. Passing south along the Pacific coast, entrance to 

 Central and South America would be had and the subsequent eruption 

 of South American forms into the north of Eocene times, would give 

 an explanation of their appearance in the Laramie fauna. This would 

 also give a rational explanation for the present existence of the Cali- 

 fornian colony. 



On the other hand if such were the history of the introduction of the 

 Viviparidw, it is difficult to see why it is not shown by a similar dis- 

 tribution to the west and south. In the same way, if the Californian 

 PleuroceridcB are derived from the former inhabitants of the Laramie 

 Sea it is difficult to understand why, when we find the two families there 

 associated under similar and evidently favorable circumstances, that 

 the Sierra Nevada mountains should have proved an insurmountable 

 barrier to the one and not to the other. The failure of the Unionidw 

 of the same fauna to effect an entrance into California is also signifi- 

 cant. 



But if we would assume that instead of coming from the orient, the 

 YiviparidcB originated in North America, where from the earliest times 

 there has been the greatest generic differentiation, and that like the 

 Camelidce among the mammals, they passed around the mountains to the 

 north and spread westward over the Behring bridge into the old world, 

 all such apparent inconsistencies would be obviated and theoretical con- 

 clusions would be in entire harmony with the known distribution. 



The present unione fauna of North America far exceeds that of any 

 other country in the abundance of its species and the almost infinite 

 diversity of shape, size and ornamentation. The questions connected 

 with the origin, differentiation and distribution of this family are there- 

 fore of peculiar interest to the American student. Like the other 

 families of non-marine mollusca the ultimate origin or. rather, point of 

 separation of these forms from the marine type, is unknown and even 

 upon theoretical conclusions naturalists are not agreed as to the probable 

 line of descent. "Although certain shells found in the Carboniferous 



