GEOLOGICAL AND GEOGRAPHICAL DISTRIBUTION 533 



The largest of these fossil Horsetails were the Calamites, which 

 have been so perfectly preserved that their structure is accurately 

 known. They were like gigantic Equiseta, from which they differed 

 in having a secondary growth of the vascular bundles of the stems, 

 like that in the Coniferae. There were also differences in the arrange- 

 ment of the sporangia. Some of these Calamites were heterosporous, 

 but heterospory was not as pronounced as in some of the Ferns and 

 Lycopods. 



The Calamites disappeared before the end of the Palaeozoic ; and in 

 the Mesozoic and succeeding formations the forms encountered all 

 belong to types closely resembling the existing genus Equisetum, 

 although many of the Mesozoic forms were very much larger than 

 any living species, and probably showed a secondary growth of the 

 stem, which is completely absent in all living species, although a 

 trace of this has been found in Equisetum telmateia. 



Sphenophyllales. Among the characteristic Palaeozoic fossils are 

 certain Pteridophytes, Sphenophyllales, which cannot be satisfac- 

 torily referred to any of the three existing classes. These, on the 

 whole, resemble most nearly some of the Calamarieae, to which they 

 are probably remotely related. A recently described fructification, 

 described under the name Cheirostrobus, is considered by Prof. D. H. 

 Scott to represent a type combining characters of the Sphenophylla- 

 les, Calamariese, and Lycopodiales. 



Lycopodiales. Like the Equisetales, the Lycopods also reached 

 a much greater development in Palaeozoic times than at present, 

 although there is not the disparity in numbers between the fossil 

 and living forms that characterizes the former class. In their 

 general characters, the Palaeozoic Lycopods closely resembled their 

 existing descendants, but most of them were of large size, often 

 becoming lofty trees, thirty to forty metres Jn height, and showing 

 a marked secondary thickening of the vascular bundles. 



Like the modern Club-mosses, the branches forked dichotomously 

 and were thickly beset with narrow leaves. These, in the very 

 characteristic genera, Lepidodendron and Sigillaria, when they fell 

 off left a scar, which persisted upon the stem, and was not unlike 

 the scars found upon the younger branches of some large-leaved 

 species of Pines. 



The cones of some of these fossil Lycopods have been preserved, 

 so that their structure is pretty well known. The cones described 

 under the name Lepidostrobus are much like those of Lycopodium 

 or Selaginella, each of the sporophylls of which it was composed bear- 

 ing a single sporangium upon its upper face. The sporangium was 

 much larger than that of any existing forms, and sometimes two sorts 

 of spores were developed. Whether heterospory was carried so far as 

 to produce seeds in any plants of this series is a disputed question. 



