GEOLOGICAL AND GEOGRAPHICAL DISTRIBUTION 225 



rosporous, but the difference between the macrospores 

 and microspores was less than in the other groups of 

 heterosporous Pteridophytes. 



The oldest fossils which can be referred to the 

 Equisetinese, occur in the Devonian rocks. They 

 increase in numbers in the overlying formations, reach- 

 ing their maximum development in the Carboniferous, 

 after which they rapidly diminish in numbers, until the 

 sole survivors of this once important group are reduced 

 to the members of a single genus. 



A very characteristic order of fossil Pteridophytes 

 is the Sphenophyllese, sometimes associated with the 

 Calamiteee, but probably better separated from the other 

 Pteridophytes as a special class now totally extinct. 

 They had slender stems with the leaves arranged in 

 whorls. The leaves were narrowly spatulate, with more 

 or less conspicuous dichotomous divisions and dichoto- 

 mous venation. The stem was traversed by a single 

 axial vascular bundle not unlike that of Lycopodium. 

 The sporangial spikes have been preserved, and it is 

 evident that the plants were sometimes heterosporous. 

 Their exact relation to the other Pteridophytes is still 

 uncertain, and further investigations are necessary to 

 determine this. 



The Lycopods also reached their greatest develop- 

 ment during the Carboniferous, and like the Equisetinese 

 these ancient forms far surpassed, both in size and com- 

 plexity, their modern descendants, none of which are 

 plants of large size, the largest being slender creeping 

 or half-climbing forms, reaching occasionally a length 

 of four to five metres. The living genera, Lycopodium 

 and Selaginella, both occur fossil, the former extending 



