What gift finally gets Helen interested in antediluvian science?

From the text:

Once a gentleman, whose name I have forgotten, sent me a collection of fossils—tiny mollusk shells beautifully marked, and bits of sandstone with the print of birds' claws, and a lovely fern in bas-relief. These were the keys which unlocked the treasures of the antediluvian world for me. With trembling fingers I listened to Miss Sullivan's descriptions of the terrible beasts, with uncouth, unpronounceable names, which once went tramping through the primeval forests, tearing down the branches of gigantic trees for food, and died in the dismal swamps of an unknown age.