32 TUE rOTOMAC OR YOUNGER MESOZOIC FLORA. 



of the time of the deposition of the Potomac material. The pecuhar 

 localization of the plant-remains causes the aggregate of different species 

 collected at the various places to be unexpectedly large. 



It is perhaps proper to say something concerning the plan I have 

 adopted in depicting these plants, for all tlie drawings were made by 

 myself The principal figures have been made of natural size, and por- 

 tions have been given enlarged to show details of nervation and other 

 significant points. In almost every case the enlai-gement is two diameters ; 

 in a very few cases, where the parts of the plant were very minute, three 

 diameters were required. Owing to the great saving in time and labor I 

 have drawn the principal figui-es simply in outline, the details omitted 

 being shown in the enlarged parts. As a rule, I have not attempted to 

 give a fac-simile of all the accidental defects and distortions shown in the 

 specimens figured, but at the same time care has been taken not to intro- 

 duce anything hypothetical. 



It has sometimes happened that in removing fossils from the rock in 

 quarrying, impressions were exposed that could not be taken up without 

 breaking them and diminishing the perfection of the specimen. Then 

 again, in transporting the material and in attempting to split up the rook 

 and work out the fossil, parts would be broken off or otherwise destroyed. 

 This was especially liable to happen in friable and tender material, which 

 at the same time had little or no regular lamination and structure. These 

 facts justify a partial restoration in drawing some of the impressions, for 

 tlie existing specimens do not in all cases fairly rejiresent what has been 

 seen. In some cases, then, missing portions have been supplied in the 

 drawings where they have been at one time seen, or where it is clear from 

 the parts still remaining what was the character of the missing ones. 

 These figures I designate as slightly restored. 



In a few cases, by taking different portions of the same plant, it has 

 been possible to give a restoration of the entire leaf, which would show 

 what were its probable shape and dimensions. This has been done in the 

 case of some of the dicotyledons found at Fredericksburg. These were 

 foiuid mostly in a very fragmentary condition, but the fragments were 

 sometimes numerous enough to show partially all portions of the leaf. 



