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 Linnaeus; 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 objects 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 



* Dr. James Edward Smith, President of the Linnean Society, &c. 



