GEOLOGICAL AND GEOGRAPHICAL DISTRIBUTION 531 



Characeae. The Characeae are known in a fossil condition in rocks 

 of various ages. The genus Chara is first certainly met with in 

 Jurassic rocks, but fossils closely resembling the characteristic 

 spirally marked spore-fruits are met with in Devonian and Carbonif- 

 erous strata. It seems probable that these really represent fossils 

 allied, at least, to the true Characeae. 



Diatoms. As might be expected, the Diatoms are found abun- 

 dantly in a fossil state, their silicious shells being almost indestruc- 

 tible. The Diatoms, if we may trust the geological record, are not a 

 very ancient group of plants. The oldest authentic record occurs in 

 the Lias, but except for two species occurring there, all of the fossil 

 Diatoms are of Cretaceous age, or more recent, and these fossil 

 Diatoms, with few exceptions, are closely allied to, or identical with, 

 existing species. 



Fungi. The occurrence of fossil Fungi has been recorded from 

 rocks of various ages, but in most cases the remains are too uncer- 

 tain to make a satisfactory determination possible. The Fungi are 

 usually found associated with other vegetable structures upon which 

 they were growing as parasites or saprophytes. 



Bryophytes 



The remains of Bryophytes, except a small number in the more 

 recent formations, are so fragmentary and uncertain as to throw no 

 light upon the geological history of the class. The extremely per- 

 ishable character of the Liverworts, and the danger of confusing 

 imprints of other plants with them, explains the almost complete 

 absence in a fossil state of structures which can certainly be as- 

 signed to this group. Of the few forms which seem to be rea- 

 sonably certain may be mentioned Marchantites Sezannensis, of the 

 Oligocene, which is allied to the living Marchantiacese. The only 

 fossils which are certainly to be referred to the Musci are all from 

 the Tertiary and later rocks, although some Palaeozoic fossils have 

 been doubtfully referred to this class. 



Fossil Pteridophytes 



The Pteridophytes are abundant in a fossil condition, and in many 

 instances so perfectly preserved as to leave no doubt as to their 

 character. Indeed, in some instances, the tissues themselves have 

 been so little changed that the cellular structure may be made out 

 with great clearness, and our knowledge of some of these extinct forms 

 is very complete. It is evident, also, from a study of these fossil 

 forms that many types have become entirely extinct, and that others 

 have left but few and degenerate descendants at the present time. 



