CTCADACE/E. 



appears to be confirmed by some specimens in tbe Binney Collection l 

 of Coal-Measure plants. In one instance this form of petiole bas 

 been found inserted on a stem of Medwttosa Leuckarti, Gb'pp. and 

 Stenz., a plant with distinctly cycadean characteristics. Probably 

 we may regard Myeloxylon as a synthetic or intermediate form 

 exhibiting cycadean and fern characters, but more nearly allied to 

 existing Cycadece than to the Filicince. In the Coal-Measure genus 

 Lyginodendron? originally described in detail by Williamson in 

 1873, we have another important link in the chain of cycadean 

 phylogeny. A revision of the English specimens of this plant, and 

 an examination of fresh material by Williamson and Scott, has 

 brought into greater prominence the clearly defined cycadean 

 features exhibited by the Lyginodendron stems. It has recently 

 been shown by these observers that Williamson's genus Kaloxylon 

 represents the root of Lyginodendron, and we have previously 

 learned that Rachiopteris aspera, Will., with its sphenopteroid 

 pinnules, is a branch of the same plant. 3 This is, again, an instance 

 of cycadean and pteridophytic characters combined in a synthetic 

 genus. The presence of secondary vascular tissue in Lyyinodendron 

 lends additional interest to this instance of fern-cycad alliance. 

 In speaking of the occurrence of diploxyloid structure in this 

 genus, Bertrand and Renault 4 regard the existence of such a type 

 of vascular bundle in the petioles of recent cycads as a remnant 

 of an ancestral structure. 



The same diploxyloid arrangement occurs on an extended scale 

 in the Permo- Carboniferous genus Poroxylon? and must be looked 

 upon as an important aid in any attempt to trace the lines of 

 development of the Cycadacea. Renault has founded the genus 

 Cycadoxylon 6 on a fragment of a silicified branch from Autun, 

 in which the structure of the wood and fundamental tissue bears 

 a distinct resemblance to a young cycadean stem. He suggests 

 that this type may find its true position between cycads and 



1 Now in the "Woodwardian Museum, Cambridge. 



2 Williamson (1, part iv.). The name was proposed hy Gourlie in 1843. 

 ("Williamson, p. 393.) See also Solms-Laubach (A.), Fossil Botany, p. 358. 



3 Williamson (1, pt. vi.), p. 684 ; also (1, pt. xiii.), p. 298. 



4 (1), P- 237. 



5 Bertrand and Renault (2). 



6 Renault (1), p. 283. 



