414 SySTEIIATIC PAlEOIfTOLOGY 



Goeppert's work was ably snpplemeiited by that of KJraus, who 

 points ont that the ustial specific characters used by students of Tascular 

 anatomy snch as the breadth of the aminal ring, width of the cell-lnmina, 

 thickness of the cell-wall, nnmber of rows of pits on the radial walls 

 of the tracheids, height of the medullary rays and their frequency, can 

 only be used with the most extreme caution on account of the individual 

 variation of these parts in difierent parts of the same plant or on 

 different individuals of the same species.^ Krause's diagnosis of Cupress- 

 inoxylon as given by Schimper - is as follows : 



"lignum stratis coneentricis distinctis, angustis; cellulis prosen- 

 ehymatosis porosis, poris magnis, rotundis, uni-vel pluriserialibus, oppo- 

 sitis; cellulis resin if eris creberrimis, ductibus resin if eris nullis, radiis 

 medullaribus simplicibus/' 



More modem studies have emphasized the uncertainties of vascular 

 anatomy as indicative of exact relationship." 



A large number of species, so-called, have been described, Schimper 

 in 1870 listing 41,* and many more have been described since Schimper's 

 list was prepared. These range in age from the Lower Cretaceous to 

 the Pliocene. 



The fossil wood and lignite of the Potomac beds was made the sub- 

 ject of a monograph by Knowlton in 1889 * which remains our only 

 source of knowledge of the anatomy of the Potomac plants. A chapter 

 is devoted to the very interesting historical development of the studv of 

 fossil wood which strangely enough, considering its abundance, is 

 not mentioned by any of the ancients, the first authentic record being 

 that of Albertus Magnus about the middle of the 13th century.'' 



Both lignite and petrified wood, either silicified or ferruginized, are 



^ See Solms-Laubacli, Fossil Botany, 1891, pp. 81-83. 



^ Schimper, Pal. Veget., tome ii, 1870, p. 374. 



' Gothan. Zur Anatomie lebender und fossiler Gymnospermen-Holzer, Abh. k. 

 preuss. geol. Landes, N. F., Heft xliv, 1905, pp. 101-103. 



* Schimper, loe. cit., pp. 374-377. 



= Knowlton, Bull. U. S. Geol. Sxirvey, No. 56, 1889. 



'^Beati Albert! Magni de Mineralibus Tractatus I, Caput VII, Opera, Lug- 

 duni, 1651, voL ii, p. 216. 



