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November 20, 1850. 

 The President in the Chair. 

 Present, fifty-one members. 



The President exhibited to the Society a number of 

 specimens, in addition to those presented at the last meet- 

 ing, of casts of Himalaya fossils, constituting the recent 

 donation from the East India Company. The specimens 

 were Crania from the families Q^iiadrumana, Carnivora, 

 Pachydermata, &c. He pointed out the distinctive charac- 

 ters of each. 



Prof. Agassiz stated that he had been of late engaged in 

 the study of the soft parts of American fresh water Mol- 

 lusks, and their relations to the shell, with the object, if 

 possible, of discovering some new characters on which to 

 base an accurate classification. 



He had found that in addition to the two muscular impres- 

 sions in the shell usually described, there are generally two or 

 more produced by muscular fibres springing from the foot, 

 which impressions in some species are confluent, in others 

 more or less distinct. Other impressions exist, produced by 

 the gills, the palpi, and the dorsal gland. Other distinctive 

 characters are observed in the arrangement of the mantle. In 

 some of the Naiades the posterior portion of the gills only is 

 found to be distended with eggs, at the breeding season ; in 

 others the whole gill is so distended. In the former of these, 

 Prof. Agassiz had found the cavity containing the eggs to be 

 limited at each end by transverse bridges beyond which the folds 

 of the gills are closely adherent to each other, and that at this 

 season the outer gill is longer than the inner, in both sexes. 

 Other characters are furnished by the gills according as they are 

 attached to the foot or the transverse muscles. Prof. Agassiz 

 said, that he had been led by these observations to a division of 



