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 
Equisetinez, 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 Sphenophyllew, sometimes associated with the 
Calamitez, 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 Equisetinee 
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 
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