460 



PENNSYLVANIA^ PERIOD 



They were probably associated in thickets and jungles, like cane- 

 brakes and bamboos. Their history may run far back, as they were 

 well differentiated in the Devonian; but their ancestry is uncertain. 

 The stems of adult calamites are so unlike those of modern horse- 

 tails that their kinship was long unrecognized, and calamites were 

 thought to be gymnosperms; but it is now known that the stems of 



Fig. 396. GROUP or FERN FRONDS: a, Neuropteris auriculata, Brgt.; b, N. 

 angustifolia, Brgt.; c, N. vermicular is, Lx.; d, Odontopteris cornuta, Lx.; e, Pccoptcris 

 imita, Brgt.; /, Dictyopteris rubelia, Lx.; g, Archceopteris bochsiana, Goepp.; h, 

 Sphenopleris splendens, Lx. 



the young plants had the same general structure as the horse-tails, 

 and that the gymnospermous features belonged to the later stages 

 of the life of the individual plants. The group is represented to-day 

 by one genus (Equisetum) and about 20 species. Its evolution 



