34 STUDIES IN FOSSIL BOTANY 



that of the simplest Coniferous woods of the present 

 day, as, for example, that of the Yew. 



There are certain other Calamarian types of stem- 

 structure, some of which have long been distinguished. 

 There is one form which became known to us by the 

 observations of Williamson, who founded a new genus 

 for it under the name of Calamopitys, but he afterwards 

 let the generic distinction drop, although the type is 

 quite distinct from the ordinary Cal'amitean structure 

 already explained. The name Calamopitys is not ad- 

 missible, as it had previously been employed by Unger 

 in a different sense, 1 and that of Artlirodendron has now 

 been substituted. The Artlirodendron type of stem is a 

 rare one. The wood, in the specimens known, is of no 

 great thickness, and the primary bundles are widely 

 separated by the principal medullary rays. The chief 

 peculiarity is in the structure of the rays, which are 

 formed, for the most part, of vertically elongated pro- 

 senchymatous cells, thus differing widely from the usual 

 parenchymatous structure of these organs ; but within 

 these primary rays are little secondary rays of paren- 

 chyma, like those of the true wood. Arthrodendron has 

 also some other peculiarities, but the complex rays 

 suffice to mark it off from the ordinary Calamites. 



There is another important type, not represented in 

 England — that of Calamodendron, — which is the most 

 complex of all (see Fig. i o). Here each of the principal 

 rays consists of a middle band of parenchyma, more or 

 less like an ordinary medullary ray, and on either side 

 of this, separating it from the wood, is a broad band of 

 fibrous prosenchyma, in which secondary rays occur. 



1 See Chapler XI. 



