34 Organisation of Fossil Plants of the Coal-measures. [May 11,. 



and it must have been from these newly-formed cells that the medul- 

 lary cylinder obtained the elements out of which to construct the 

 additional vessels, the increase of which has been shown to be the 

 invariable accompaniment of the growth of the branch. As might be 

 expected, the growth of the vascular cylinder or medullary sheath 

 could only have been a centripetal one. 



A new form of Halonia from Arran is described. Instead of its 

 central portion consisting, as in previously described examples, of the 

 usual Lepidodendroid medulla, surrounded by a vascular cylinder, it 

 consists of a solid axis of vessels, resembling in this respect all the 

 very young Lepidodendroid twigs previously described from the same 

 iocality. Many recently obtained specimens of Lepidodendroid 

 branches sustain the author's previous observations that all examples 

 from Arran having less than a certain diameter have the solid axial 

 bundle ; whilst all above that diameter have a cylindrical vascular 

 bundle enclosing a cellular medulla. The first type commences with 

 the smallest twigs, and is found increasing gradually up to the dia- 

 meter referred to. The second type begins where the other ends, and 

 increases in diameter until attaining the dimensions of the largest 

 stems, in none of which does the solid bundle reappear. Halonial 

 branches have not hitherto been described attached to the branches of 

 any true Lepidodendron, though, in 1871 (Memoir, Part II), the 

 author gave reasons, based upon organisation, for insisting that Halonia 

 was a fruit-bearing branch of a Lepidodendroid tree. This con- 

 clusion was sustained by Mr. Carruthers in 1873 in his description of 

 a branch belonging to a Lepidophloios. The author now figures a 

 magnificent example, from the museum of the Leeds Philosophical 

 Society, of a dichotomous branch of a true Lepidodendron of the 

 type of L. elegans and L. Selaginoides. In this specimen every one 

 of the several terminal branches bears the characteristic Halonial 

 tubercles. The leaf-scars of these latter branches have the rhomboid 

 form once deemed characteristic of the genus Bergeria, whilst those of 

 the lower part of the specimen are elongated as in L. elegans, &c. 

 These differences are not due to their appearance in separate cortical 

 layers of the branch, but to the more rapid growth in length of its 

 lower part compared with its transverse growth. 



The author throws some additional light upon the structure of 

 Sporocarpum ornatum described in Memoir, Part X, as also upon the 

 nature of the development of the double leaf-bundles seen in trans- 

 verse sections of the British Dadoxylons, described in Memoir IX. 

 After a prolonged but vain search for a similar structure amongst the 

 twigs of the recent Conifers, the author has at length found them in 

 the young twigs of the Salisburia Adiantifolia. Sections of these twigs 

 made immediately below their terminal buds exhibit this geminal 

 arrangement in the most exact manner. Pairs of foliar bundles are 



