456 PALEOBOTANY. 



(Dawson, Acadian Geology, p. 441). Besides the true 

 Calamites, the Carboniferous rocks have also yielded the 

 remains of the genus Equisctites, which differed from the 

 Calamites, and agrees with the existing Horse-tails in having 

 sheaths at the joints. 



Calamites often attain a comparatively gigantic size — 

 twenty feet or more in length ; and though they generally 

 occur as prostrate and flattened stems, they are not uncom- 

 monly found in an erect and uncompressed condition, stand- 

 ing as they grew. The fossils known as Asterophyllites have 

 been referred to Calamites, of which they are sometimes 

 supposed to constitute the foliage ; but this opinion is not 

 accepted by high authorities. 



c. Calamodcndron. — A good deal of the confusion which 

 has prevailed as to the true nature of Calamites appears to 

 have arisen out of the uncertainty which has long prevailed 

 as to the true nature of the problematic fossils now gen- 

 erally referred to the genus Calamodcndron. As ordinarily 

 found, Calamodendra present themselves in the form of 

 jointed and longitudinally-ribbed cylindrical stems, which are 

 hardly separable from Calamites, except that they show no 

 " areoles," or points whence leaves or branchlets have been 

 given off. From the examination, however, of complete 

 specimens, it has been shown that Calamodendron, as thus 

 constituted, is really nothing more than the cast of the pith 

 or medullary cavity of a complex woody stem, thus resem- 

 bling in its nature the fossils known as Sternhergia. Eound 

 the internal axis thus constituted there is found in perfect 

 examples a thick woody envelope, composed of ligneous 

 wedges arranged concentrically and separated by intervening 

 tracts of cellular tissue (or " medullary rays "). The ex- 

 ternal surface of the stem is not known, but the woody 

 wedges are stated to consist of " elongated cells, and porous, 

 discigerous, or pseudo-scalariform tissue." The ailinities of 

 Calamodendron are uncertain. It is regarded by different 

 authorities as belonging to the Gymnospermous Exogens or 

 to the Acrogens, or as a connecting form between these 

 groups. Upon the above view, it is necessary to distinguish 

 very carefully between Calamodendron and Calamites, the 



