XXX] MEDULLOSA 95 



Lower Coal Measures of Colne, Lancashire. It agrees in essential 

 features with Medullosa anglica, but differs in the following parti- 

 culars: the linear dimensions of the stem are about one quarter 

 those of a typical stem of the older species ; the leaf -traces possess 

 little or no secondary xylem and the relatively large decurrent 

 leaf-bases have a narrower and simpler hypoderm. The stem 

 has a tri-stelar vascular system enclosed in a ring of internal 

 periderm, and each stele (3 mm. in diameter) consists of a roughly 

 triangular strand of reticulate tracheids and a small amount of 

 scattered parenchyma. The protoxylem is either exarch or, as 

 in M. anglica, mesarch, the exact position being difficult to deter- 

 mine in the available material. The secondary xylem closely 

 resembles that of M. anglica. 



Scott suggests the possibility that Alethopteris decurrens may 

 be the foliage of Medullosa pusilla. It is possible that there is 

 no specific difference between M. pusilla and M. anglica, but 

 on the present evidence the employment of a distinctive name 

 is desirable. 



Medullosa centrofilis de Fraine. 



This species was founded by Miss de Fraine 1 on a petrified 

 stem from the Lower Coal Measures of Lancashire. The maximum 

 diameter of the flattened stem including four decurrent leaf-bases 

 is 5 cm. The vascular system consists of an outer group of four 

 steles, reduced to three by fusion in the upper part of the specimen, 

 enclosing a central smaller stele or star-ring (fig. 417). It is the 

 presence of the star-ring that distinguishes this type from the 

 other two British species and forms a connecting link with certain 

 continental Medullosae. The peripheral steles agree with the 

 steles of M. anglica but, as in M . pusilla, there is some doubt as to 

 the exarch or mesarch position of the protoxylem. In the struc- 

 ture of the xylem the central stele conforms to the rest of the 

 vascular system and a strand of protoxylem is preserved that is 

 almost certainly exarch. There is evidence that the peripheral 

 steles occasionally anastomose, but the central stele follows an 

 independent course at least in the piece of stem examined. Leaf- 

 traces are furnished by the primary xylem of the outer steles, and 



1 De Fraine (14). 



