412 VEGETATION OF THE CARBONIFEROUS PERIOD 



the longer is in both three times branched, and bears a sorus on the first 

 or right-hand branch. But whereas in A ( Cyclopeltis Presliana, Sm.) 

 the said first right-hand branch or venule is not produced to the margin 

 of the frond, and bears its sorus on the point, in B ( Cycl. semicordata, 

 Sm.) the same venule is produced to the margin, and has the sorus on 

 its middle. Now this arises from no difference of specimens, but of 

 species, and it is the only character by which a plant exclusively in- 

 habiting the Old World can be distinguished from one confined to the 

 New. 



Though such an anomaly as that of a growing fern bearing fronds 

 with two different types of venation is readily recognized amongst 

 recent plants whose whole frond is presented to the eye, it is not so 

 in the petrified specimen, of which isolated fragments must be classed 

 under different genera according to the arrangement of the veins. 

 This is probably a fertile agent in dismembering genera, an error no 

 less grave than that to which so slight a character as that distinguishing 

 Cyclopeltis Presliana from C. semicordata, would, if overlooked, lead ; 

 for hence we might specifically unite the widely different ferns of 

 two more differently circumstanced countries than Europe and North 

 America. 



Lastly, many recent ferns are of so dense and leathery a texture 

 that their venation cannot be ascertained without much trouble, arising 

 from the necessity of macerating the frond and dissecting out the veins. 

 When such are petrified, they cannot be referred with propriety to any 

 previously established genus, the characters afforded by texture, and 

 comparative denseness of substance, being of specific importance only. 



The conclusion to be drawn from the above resume of the value of 

 the characters drawn from the venation of fossil ferns is that the pre- 

 vailing tendency of dwelling too much upon it leads to the dismem- 

 berment of species, and placing the individual parts under different 

 genera. 



Outline and Division of Fronds. — A character considered next in im- 

 portance to venation, both amongst recent and fossil ferns. It is seldom 

 of more than sectional or specific value, and frequently not that, the 

 species of a genus being frequently grouped into those with simple 

 fronds, those with the fronds lobed, deeply divided, once, twice, and so 

 forth. 



In some cases of living ferns even the amount of variation is very 

 startling, for instance the Polypodium pustulatum (Fig. 9), where the 

 fronds are sometimes entire, at others deeply lobed ; or still more 

 remarkably in the case oiLindscea cor data (Fig. 10, p. 414), whose fronds 

 vary so extraordinarily, that did the various forms occur petrified, and 

 detached from the parent plant, it would be impossible to deny specific 



