274 THE AMERICAN NATURALIST. [Vor. XXXIX. 
by Equisetum silvaticum, there is an inner as well as an outer 
endodermis, common to the whole circle of bundles. Other dif- 
ferences are the presence or absence of branches at the nodes, 
and the development in certain species, like Eguisetum arvense, 
of special sterile and fertile shoots. 
FossıL EQUISETALES. 
The earliest fossils belonging to the Equisetales belong to the 
genus Archzocalamites, which in many respects was very simi- 
lar, as regards both its anatomy and fructification, to the genus 
Equisetum. Fossils of undoubted equisetinean affinities abound 
in the later Palaeozoic formations, being especially abundant in 
the coal measures. Of these, the genus Calamites was especially 
conspicuous. These were much more highly organized than the 
living Equiseta, which more nearly resemble the more ancient 
Archeocalamites, Fossils closely allied to the genus Equisetum 
occur frequently, however, in later formations, being abundant 
in the earlier secondary rocks. 
Undoubtedly related to the Equisetales was the characteristic 
Paleozoic group of Sphenophyllales, but there is a good deal of 
diversity of opinion as to the possible connection of the Spheno- 
phyllales with other groups of pteridophytes besides the Equise- 
tales. 
The genus Equisetum has been the subject of repeated 
investigation, and we are now well informed concerning pretty 
much all its structural details and developmental history. The 
conclusions drawn from a study of these data by different ob- 
servers, however, are by no means all in accord. This differ- 
ence is especially marked in the attempts to decide the affinities 
of the Equisetales with the other pteridophytes. It is mainly 
for the purpose of examining and comparing these divergent 
views that the present paper has been prepared. 
Briefly stated, the following are the different views held as to 
the relationships of the Equisetales : — 
I. They are allied to the lycopods. 
2. They are allied to the ferns. 
3. They are allied to neither of the other existing classes of 
pteridophytes, but have had an independent origin. 
