XXX] MEDULLOSA 89 



of a solenostele or the development of an apparently complex 

 dictyostele are features independent of the leaf and leaf-traces. 

 In the structure of the secondary xylem and phloem and in root- 

 structure Medullosa agrees with recent Cycads. The genus is in 

 short a generalised type with filicinean and cycadean affinities. 

 In the possession of seeds borne on modified pinnae of compound 

 fronds, Medullosa resembles both Cycas and the Lyginopterideae. 

 The seeds exhibit a fairly close agreement with those of Lygino- 

 pteris, Heterangium and recent Cycads, but they appear to have 

 advanced further towards the cycadean type than is the case 

 with the closely related seeds of the Lyginopterideae. The 

 microsporophylls are very imperfectly known but they were 

 undoubtedly much less advanced and more fern-like than the 

 megasporophylls . 



The genus Medullosa is recorded from the Permian strata of 

 Saxony, France, and Bohemia 1 ; also from the Coal Measures 

 of England, and the discovery of petrified petioles of Myeloxylon, 

 the type borne on Medullosan stems in European species, may 

 be taken as evidence of the existence of the genus in North America 

 during the Carboniferous period 2 . 



The name Medullosa was applied by Cotta 3 to three types, 

 Medullosa elegans, M. stellata, and M. porosa, from the Roth- 

 liegende of the Chemnitz district. The first of these was recognised 

 by Brongniart 4 as a distinct genus for which he proposed the 

 designation Myeloxylon and this was afterwards identified by 

 Renault, Williamson, and other palaeobotanists as a petiole and 

 not a stem. Further reference is made to Myeloxylon on a later 

 page. Cotta spoke of Medullosa as the most puzzling of the 

 genera dealt with in his 'Dendrolithen,' and in spite of the many 

 additions to our knowledge the position of this Palaeozoic genus 

 is still a fertile source of speculation. The generic designation 

 Medullosa is applied to stems, with or without petioles; petioles 

 or rachises of fronds that frequently occur apart from stems are 

 referred to the genus Myeloxylon. The leaves of Medullosa 

 include several well-known species of Carboniferous and Permian 

 genera such as Alethopteris, Neuropteris and others that have in 



1 Solms-Laubach (97) PI. vi. fig. 3. 2 Penhallow (97). 



Cotta (32) B. p. 59. 4 Brongniart (49) A. p. 57. 



