l8 9i-J Fossil plants of the Coal-measures. 173, 



name Rachioptcris Grayii, while the section of a hairy branch- 

 ing stem or rhizome, also from Halifax, resembling in 

 many respects the young branches of living Marsilese, is 

 named R. Jiirsnta. Two roots, from Halifax and Oldham, 

 showing tincture minutely, are called Rhizonium vcrticilla- 

 turn and R. reticulatnm; and a third, equally remarkable 

 in structure, whose features resemble those seen in the 

 cortex of Asteropliyllites Willi amsonis, is named R. lacn- 

 nosum. The examination of new specimens of Cala- 

 mostachys Binncyana has enabled the author to fill 

 two gaps in the knowledge of the structure of that 

 interesting form of strobilus, viz. the distribution of the 

 vascular bundles of the central axis, and the peripheral ter- 

 mination of the sporangiophores. Sections of a fertile 

 strobilus through the latter show a thickened distal 

 end, occupied by clusters of tracheids concentrated in the 

 region where each sporangium is organically attached, show- 

 ing '* that these peripheral terminations of the sporangiophores 

 approach even nearer than they were previously known to do- 

 to those of the living Equisetum, in corresponding parts of 

 which similar clusters of tracheids exist." 



Part XVI is devoted to studies of the mode of branching, 

 the formation of the medulla, and the method of exogenous 

 growth as seen in a number of Lepidodendra, several of which 

 are here first described. The author considers the ordinary 

 mode of branching in this genus to have been dichotomous,. 

 with a perfect dichotomy of the medullary vascular cylinder, 

 but that only a segment was cut off from that vascular bundle 

 when the branch was of a special kind, "characterized by an 

 arrested development," such as is represented by the tuber- 

 cles of Halonia, or the scars of Ulodendron in which the 

 arrested branches supported Lepidostrobi, both of the latter 

 genera being only "conditions" of various Lepidodendroid 

 genera. 



He finds that the germs for the gradual formation of a me- 

 dulla in the center of a vascular bundle which previously con- 

 tained no traces of cellular structure were furnished by the 

 procambium from which, in the youngest twigs, the entire 

 bundle originated. The first one or two medullary cells, 

 formed in the centre of the bundle of tracheids, increased by 

 the ordinary meristemic process of enlargement and fission. 

 The internal tension produced by the enlargement of the med- 



