174) PROF. W. C. WILLIAMSON ON THE STRUCTURE 



In addition^ I would suggest the establishment of the 

 new genus Calamopitus {/cdXafjL0<;-'7rLTv<;) for those forms 

 in which the woody elements consist of reticulated vessels 

 associated with medullary rays^ and having verticils of 

 medullary radii near. the nodes*. 



The exact value of these medullary radii, as indicative 

 of a generic distinction, remains to be ascertained, as well 

 as the extent to which they are associated with reticulated 

 vessels. If it should be found that they occur in Calamites 

 with scalariform vessels, or if Calamites having none but 

 reticulated vessels exist without traces of verticillate me- 

 dullary radii, then it may be necessary to abandon the 

 the term Calamopitus, and associate the whole series as 

 variable examples of Brongniart^s genus C alamo dendr on. 

 At present, however, we know that neither Binney^s nor 

 Unger's plants possess the verticillate radiating prolonga- 

 tions of the pith. They are also absent from silicified 

 Calamites from Autun, of which M. Brongniart showed 

 me fine specimens many years ago, and which, he now in- 

 forms me, are identical with Mr. Binney^s plants. Dr. 

 Dawson says that true Calamites " can always be distin- 

 guished ^^ p. e. from piths of C alamo dendr a'\ " by the scars 

 of the leaves or branchlets which are attached to the 

 nodes."*^ But my plants indicate precisely the opposite 

 conclusion to this, viz. ist, that such scars appear to 



* I do not altogether like this ari'angement, because I have the strongest 

 doubts respecting the existence of an equisetiform type of Calamitea part 

 from what Mr. Binney has described. I should have preferred applying 

 the old generic name Calamites to Mr. Binney's plants, and assigning Bron- 

 gniart's term Calamodendron to my own. But when an author founds a 

 genus, he is entitled to define it, and M, Brongniart has distinctly identified 

 Calamodendron with a vascular zone consisting of scalariform vessels and 

 true medullary rays. The plan proposed presents the further difficulty 

 that, if Mr. Binney's specimens are really deprived of medullary rays, that 

 important characteristic feature will exclude them from Calamodciidron, and 

 involve the necessity for establishing an additional genus for their reception. 

 But as I regard this as a doubtful point, I am not, at present, prepared to 

 separate them from Calamodendron. 



