90 MEDULLOSEAE [CH. 



recent years been transferred from the Filicales to the Pterido- 

 sperms. In a few instances seeds have been found in organic 

 connexion with Medullosean foliage, and there can be no reasonable 

 doubt that Trigonocarpus, some forms of Rhabdocarpus, Pachytesta, 

 and other seeds represent the integumented megasporangia of 

 Medullosa or some closely allied genus. 



Before attempting to summarise the salient features of 

 Medullosa a description of a few selected types will serve to place 

 us in a better position to consider the genus as a whole. The 

 British species are placed first on the ground that they are both 

 geologically the oldest though, historically, the most recently 

 described, representatives of the genus; and in the organisation 

 of the stem they are simpler than the continental species. Their 

 resemblance to Heterangium serves to some extent to bridge the 

 gap between the majority of species of Medullosa and the simpler 

 types of Pteridosperms represented by Heterangium and Lygino- 

 pteris. 



Medullosa anglica Scott 1 . 



Prior to the discovery of this species the genus Medullosa 

 had not been recorded from Britain. A section in the Williamson 

 collection recognised by Scott as that of a Medullosa had been 

 identified by Williamson as a large Heterangium stem. An 

 undescribed specimen was found by Arber 2 in the Binney collection 

 at Cambridge which afforded some additional information as to 

 the structure of the roots. 



The specimens on which Scott's thorough description is based 

 were obtained by Messrs Wilde and Lomax from the Lower Coal 

 Measures of Lancashire. The stem of this oldest species has the 

 habit of a tree-fern and is almost completely invested by the 

 stout decurrent bases of the petioles of large spirally disposed 

 compound fronds with a phyllotaxis of f , the leaves of the same 

 orthostichy being separated from one another by a vertical distance 

 of approximately 10 cm. 



A transverse section of a slightly flattened stem is shown in 

 fig. 416, A, the bases of three petioles give to it an angular form. 

 Its dimensions are approximately 10 x 4 cm. The ground-tissue 

 1 Scott (99). 2 Arber, E. A. N. (03). 



