2 THE POTOMAC OR YOUXGER MESOZOIC FLORA, 



shipped, unpacked, and worked out of the rough material with my own 

 hands. The specimens have in consequence iiever passed from under 

 my direct supervision. The ph\nts were collected at intervals, and 

 repeated visits were paid to the more important localities in order to 

 secure new material which might throw light on imperfect specimens pre- 

 viously obtained. This method of collection causes delay and imposes 

 much additional labor, but it has many advantages. 



In the first place, tlie exact locality and geolog'ical horizon of a specimen 

 can be fixed, a matter of great importance in determining the diffusion of 

 species, and in the discover)- of connecting links between the local floras. 

 In the Potomac formation, owing to the peculiar facies of some of these 

 floras, connecting links are of unusual importance. 



A much greater advantage is obtained in the increased number of sig- 

 nificant parts of plants selected and in the greater opportunity afforded 

 for the detection of new species. 



When the material is collected by the person who will describe it special 

 attention is jiaid to the significant portions of fossils. In the case of plant- 

 fossils, parts witli tlie gi'eatest structural significance are often not as con- 

 spicuous as those with less meaning. They may also be poorly preserved. 

 They would be rejected in such cases by a collector unacquainted witli the 

 science they illustrate, especially if the amount of material were large. 



The circumstances attending my collection of these plant-fossils were 

 most favorable for thoroughness of exploration and selection. The search 

 for these impressions extended over several years. During the summer 

 material was obtained from different localities, and in autumn and winter 

 this was worked over, studied, and drawn, the drawings in all cases being 

 made by myself. 



It often happened in this study that a number of fragments unlike 

 the rest were found which were too imperfect to be made out, and tliis led 

 me in several cases to make repeated visits to the same locality, in the 

 hope of obtaining more complete specimens of the undetermined species. 

 The renewed search often met with success. It will be easily seen that a 

 collection made under such circumstances has a completeness and value 

 not otherwise attainable. 



