104 UNITED STATES GEOLOGICAL SURVEY— TERTIARY FLORA. 



This is remarked especially in the two specimens (figs. 2 and 5) which I have 

 described as representing Lemna scutata, Daws., but which seem positively 

 referable to this species; the first by its nervation from the center, appearing 

 as if a leaf, like that of fig. 1, had been inflated and compressed, and its 

 pedicel flattened to the central ])art, wherefrom the veins are diverging around, 

 and in fig. 5, which, with the outlines of the larger leaves, has its primary 

 veins ascending from the pedicel to the borders, scarcely divided as yet, on 

 account of its incipient development. As the specimens are very numerous, 

 distinct, and the leaves compressed and imbedded into the stone in various 

 directions, it is not surprising to see this diversity of forms, which, however, 

 is merely casual, and which seem explainable in comparing the fossil 

 leaves to those of Pistia spathulafa, Michx., of the swamps of Louisiana. 

 These leaves have the same obovate shape, and the same type of nervation, by 

 inflated primary veins diverging from the point of union of the pedicel with 

 the lamina, and dividing upward in an irregular dichotomy, forming by cross- 

 nervilles an areolation similar to that of the fossil species. Moreover, most 

 of the leaves of the living plant, especially the old ones, bear on the under 

 surface an inflated spongious coating, which covers them from the base to 

 above the middle, especially along the primary veins, and which is exactly 

 similar to that observable on the lower surface of the fossil leaves. It is true 

 that the black lines encircling the intumescence are not remarked in leaves of 

 Pistia spathulata. But they may be traced by folds caused by compression, 

 the folding following of course the border of the part inflated by the peculiar 

 deposit of the under side, which seems formed by an agglomeration of radicles 

 and of their detritus by decomposition. In some of the fossil leaves, as in 

 fig. l,for example, the disconnection of the nervation along the lower rim of 

 the flat border is scarcely noticeable, and, though more distinct in fig. 3, a 

 slight folding along the rim would sufficiently account for it. It is not so 

 easy to explain tlie central appearance of the pedicel or base of the leaf of 

 fig. 2, just in the central part of an exactly round outline, if this specimen 

 represents a leaf of the same kind. This could be done only by supposing 

 that the lower part of the leaf with its pedicel has been folded up, com- 

 pressed, and eflTaced by maceration, leaving only the space marked in the upper 

 part of the leaf as trace of its existence. The lower part has not any veins, 

 while the other half has them corresponding in size and mode of ramification 



