LAEAMIE TAUNA. 211 



the oceanic basins, and the indraught of fresh water from its drain- 

 age area, progressively lost their salinity, becoming more and more 

 fresh with the advance of time. Many or most of the moUusoan 

 types that flourished during the period of greatest salinity slowly 

 disappeared from the region — through extermination or migra- 

 tion — the result of an innate incompetence to adjust themselves 

 to the changing conditions of their surroundings. Others, more 

 fortunate, by slow degrees accommodated themselves to the newly- 

 imposed conditions, and found a congenial home in harmonic as- 

 sociation with such forms as the inflowing fresh water may have 

 thrown in with them, and which were in their way undergoing a 

 reversed modification. That this must have been the actual history 

 of at least a considerable part of the Laramie region there can be 

 no question, seeing what a remarkable commingling of fresh water 

 and marine, or brackish water, molluscan types is there exhibited. 

 Thus, we have frequent associations in the same stratum of the 

 genera Corbicula, Corbula, Unio, Neritina, Vivipara, and Gonioba- 

 sis, or Corbicula, Corbula, Ostrea, and Anomia, and, as Dr. White "* 

 informs us, '' the commingling of brackish - water [Ostrea, Ano- 

 mia, Corbula] and fresh-water forms occurs in some portions of the 

 Laramie deposits under such conditions as to compel the belief 

 that some of them at least lived and thrived together." In further 

 association with these forms are representatives of the genera Leda, 

 Pectunculus, and Odontobasis, which, otherwise, are known only 

 from marine deposits. 



It is true that, in the case of the Laramie fauna, we have no 

 evidence proving that any of its distinctive fluviatile types of or- 

 ganisms have been descended by modification from marine forms, 

 inasmuch as all of them may have been primarily introduced into 

 the sea by the inflowing streams. That this was the case with 

 many of the genera is indisputably shown by their great antiquity, 

 compared with that of the fauna of the Laramie lake-basin. But it 

 is by no means unreasonable to assume, seeing how very hetero- 

 geneous in its character was the fauna of the period, that, had the 

 freshening and desiccation of the primitive inland lakes been less 

 rapid, a complete transformation of marine into fresh-water types 

 with^ the retention of permanent characters, might have been 

 brought about. Such transformation appears to have taken place 

 in the case of a portion of the molluscan fauna of Lake Baikal, 



