CH. IX] FOSSIL EQUISETALES. 255 



calamites to the Calamarieae ; such a division rests in part on 

 assumption, and cannot be considered final. When we attempt 

 to deiine the Equisetales and the two families Equisetaceae and 

 Calamarieae, we find ourselves seriously hampered by lack of 

 knowledge of certain important characters, which should be 

 taken into account in framing diagnoses. There is little harm 

 in retaining provisionally the two families already referred to, if 

 we do not allow a purely arbitrary classification to prejudice our 

 opinions as to the affinities of the several members of the Equi- 

 setales. 



The Equisetaceae might be defined as a family including 

 plants which were usually herbaceous but in some cases arbores- 

 cent, bearing verticils of leaves in the form of sheaths more or 

 less deeply divided into segments or teeth. The strobili were 

 isosporous and consisted of a central axis bearing verticils of 

 distally expanded sporophylls with sporangia, as in Equisetum. 

 The genus Equisetites might be included in this family, but it 

 must be admitted that we know next to nothing as to its 

 anatomy, and we cannot be sure that the strobili were always 

 isosporous. 



The genus Schizonevra is too imperfectly known to be 

 defined with any approach to completeness, or to be assigned to 

 a family defined within certain prescribed limits. Fhyllotheca 

 is another genus about which we possess but little satisfactory 

 knowledge ; we are still without evidence as to its structure, 

 and the descriptions oi the few strobili that are known are not 

 consistent. Recent work points to a probability of Phyllotheca 

 being closely allied to Annularia, a genus included in the 

 Calamarieae, and standing for a certain type of Calamitean 

 foliage-shoots. 



In comparing the Calamarieae with the Equisetaceae, the 

 alternation of sterile and fertile whorls in the strobilus, and the 

 free linear leaves at the nodes instead of leaf-sheaths are two 

 characters made use of as distinguishing features of the genus 

 Calamites as the type of the Calamarieae. On the other hand, 

 the strobili of Phyllotheca appear to agree with those of Cala- 

 mites rather than with those of Equisetum, and strobili of 

 Archaeocalamites have been found exhibiting the typical 



