250 On the Fossil Plants of the Coal-measures. [Mar. 26, 



thick layer, of coarse prosenchyma, and which has evidently been hard and 

 woody, as in many of the recent Adiantiums. Towards the upper part 

 of the petiole the vascular bundle becomes distinctly consolidated into a 

 single cluster of crucial form ; it then passes into a somewhat trifid form, 

 and ultimately into a small cylindrical one. This petiole has branched 

 much more freely than any of the others described. Two of the extre- 

 mities of the crucial arms of the vascular bundle become first enlarged 

 and then detached as two secondary bundles, which generally have an 

 irregularly triangular transverse section, with long arms to the triangle. 

 These triangular bundles are altogether different from the central axis of 

 Asterophyllites described in a preceding memoir. The ultimate subdivisions 

 of these secondary branches look more like the terminations of cylindrical 

 rootlets than of petioles which fact, combined with the circumstance that 

 no traces of leaflets have been found associated with any of these ultimate 

 twigs, renders the petiolar nature of this plant open to question, though 

 the arguments in favour of its being a branching fern-petiole preponderate 

 over those which militate against that conclusion. The author designates 

 this plant Rachiopteris Oldhamia. 



The next plant described is an exquisitely beautiful petiole from Burnt- 

 island, to two detached portions of which the author has already assigned 

 the names of Arpexylon duplex and A. simplex*, but which two forms he 

 now proves to belong to the same plant. In the matured petiole the 

 vascular bundle is always a double one. There is a central bundle, 

 exhibiting a transverse section shaped like an hour-glass, one side of 

 which is truncated and the other rounded, with a free, narrow, crescentic 

 band at the more truncate of its enlarged extremities. At each of these 

 extremities of the central bundle there is a longitudinal groove, which is 

 shallow on the truncated side nearest to the crescentic bundle, but so sur- 

 rounded by small vessels at the opposite convex side as often to become 

 converted into a longitudinal canal. The hour-glass bundle always reap- 

 pears in various specimens under the same aspect ; but the crescentic one 

 divides into two lateral halves, and the ends of each of these two subdivided 

 parts curl under their more central portions. We thus obtain two of the 

 crescentic structures previously designated Arpexylon simplex. These 

 crescents are traced outwards through the bark to lateral secondary rachides. 

 The vessels thus detached from the truncated side of the central hour- 

 glass bundle now reappear at its opposite and more convex side, whence, 

 in turn, they again become detached ; so that the truncate surface with 

 its crescentic appendage, and the more oblate one with its almost closed 

 canal, have alternately reversed their positions in the petiole as each 

 secondary rachis was given off. Alternating distichous tertiary rachides 

 spring from these secondary ones. 



Two plants which appear to be identical with those described by M. 

 Kenault, under the names of Zygopteris Lacattii and Z.bibractiensis, are next 

 * Proceedings of the Royal Society, yoL xx. p. 438. 



