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LETTER XLIV. 
GREAT DIFFICULTY OF ASCERTAINING EVEN THE GENERA OF THE 
PLANTS WHICH ARE THUS PRESERVED DORSIFEROUS PLANTS 
AND CACTI MOST COMMON. 
Aware of the difficulty of determining, from little more than the 
outlines of a leaf, the species, or even genus, of the plant to which 
it belonged; and also well knowing that the greater part of the 
remains of plants, thus preserved, have been acknowledged to have 
belonged to plants not known to botanists; I anxiously sought, 
and was happy in obtaining, the aid of a gentleman* whose general 
botanical knowledge has given him high rank among the disciples 
of Linnffius ; and whose particular knowledge, respecting the dorsi- 
ferous plants, would stamp considerable authority on any opinion 
he should offer respecting this order, which comprehends by far the 
greatest number of the obects of our present inquiry. But, though 
possessing all the knowledge within the reach of an European 
botanist, the close examination which he kindly made of the vege- 
table remains depicted in Plate IV. and V . would only allow him to 
give an opinion on very few of them. These fossil remains of 
vegetables are, he observes, a sort of botanical riddles ; and, with 
respect to those which appear to be ferns, the difficulty of deter- 
mining to what species the several impressions may be referred, is 
augmented by there being so many things which they may be, and 
so many things which they nearly resemble, without being the same. 
Of the figures represented in Plate IV. he was of opinion, that 
* Dr. James Edward Smith, President of the Linnean Society, &c. 
