a single row of cells which are three to five times higher than broad, as seen in tangential longitudinal section. Infranodal canals occur in some examples of _Arthropitys_. [Sidenote: ARTHRODENDRON.] In the subgenus _Arthrodendron_, a type of stem first recognised by Williamson and named by him _Calamopitys_[632], the principal medullary rays consist of _prosenchymatous cells_ (_i.e._ elongated pointed elements) and not parenchyma. These elongated elements are not pitted like tracheids, and they are shorter and broader than the xylem elements. In some examples of this subgenus the primary rays are bridged across at an early stage by the formation of secondary interfascicular xylem, and in others they persist as bands of ray tissue, as in _Arthropitys_. Other characteristics of _Arthrodendron_ are the abundance of reticulated instead of scalariform tracheids in the secondary wood, and the large size of the infranodal canals. Fig. 83, _D_ represents part of a transverse section of _Arthrodendron_; in this stem the rays have been occupied by interfascicular xylem at a very early stage of the secondary growth. The section from which fig. 83, _D_ is drawn was described by Williamson in 1871; the complete section shows about 80 carinal canals and primary xylem groups. The prosenchymatous form of the principal medullary rays is seen in fig 83, _C_, and the reticulate pitting on the radial wall of a tracheid is shown in fig. 83, _B_. Fig. 83, _A_ illustrates the large infranodal canals as seen in a tangential section of a stem. The same section shows also the course of the vascular bundles characteristic of _Calamites_ as of _Equisetum_, and the position of outgoing leaf-traces is represented by unshaded areas in the black vascular strands. [Illustration: FIG. 83. _Calamites_ (_Arthrodendron_). _A._ Tangential section (diagrammatic) showing the course of the vascular strands, also leaf-traces and infranodal canals. _B._ Radial face of a tracheid. _C._ Prosenchymatous elements of a principal medullary ray. _D._ Transverse section of the wood. (After Williamson.) No. 36 in the Williamson Collection.] The subgenus _Arthrodendron_ is very rarely met with, and our information as to this type is far from complete[633]. The third subgenus _Calamodendron_ has not been discovered in English rocks, and our knowledge of this type is derived from French and German silicified specimens[634]. There is the same large hollow pith surrounded by a ring of collateral bundles with carinal canals, as in the two preceding subgenera. The tracheids are scalariform and reticulate, and the secondary medullary rays consist of rows of parenchymatous cells which are longer than broad, as in _Arthropitys_ and _Arthrodendron_. [Illustration: FIG. 84. _Calamites_ (_Calamodendron_) _intermedium_, Ren. Transverse section through two vascular bundles. _a_, _a_, xylem tracheids, _b_, _b_, bands of prosenchyma, _c_, medullary ray. (After Renault.)]