108 MEDULLOSEAE [CH. 



but this feature is shown only in one of Cotta's figures, and Weber 

 and Sterzel 1 point out that a doubling of the hypoderm zone 

 may be caused by accidental juxtaposition of two faulted pieces 

 of peripheral tissue. The drawing reproduced in fig. 420, A, 

 shows the structural plan of an unusually large petiole from the 

 Permian of Autun : a portion of the outer tissue is seen in fig. 418, A. 

 The vascular bundle, fig. 420, C, from a Millstone Grit specimen 2 , 

 shows the centripetal nature of the xylem and fragments of phloem 

 in the outer half of the bundle, with imperfectly preserved fibres 

 abutting on the xylem. The characteristic hypoderm is shown 

 also in fig. 418, A ; the double xylem strand on the left illustrates 

 a common feature caused by the branching of vascular bundles. 

 Several secretory canals are scattered in the ground- tissue. The 

 pinnules of Myeloxylon radiatum, or at least of some specimens, 

 have been shown by Eenault to be of the Neuropteris type. 



Myeloxylon Landrioti (Renault) 3 . 



In this species the distinguishing feature is the occurrence of 

 the hypodermal stereome in the form of circular, oval, or reniform 

 strands in place of the radial plates of M. radiatum. It is this 

 form of petiole that was borne by the stems of Medullosa anglica 

 and M. LeucJcarti. In M. anglica the pinnules (fig. 420, D) are 

 of the Alethopteris type, almost certainly A. lonchitica. Renault 

 and Zeiller have described French specimens of Myeloxylon 

 Landrioti bearing pinnules like those of Alethopteris aquilina and 

 A. Grandini. 



Myeloxylon topekense (Penhallow). 



The occurrence of Myeloxylon petioles in the New World 

 was recorded by the late Prof. Penhallow 4 who founded this 

 species on some imperfectly petrified specimens from Upper 

 Carboniferous strata at Topeka, Kansas. Enough material was 

 available to show the Myeloxylon characters, but the preservation 

 is too imperfect to admit of a complete diagnosis. The hypodermal 

 stereome shows a tendency to form tangentially extended strands 

 in place of the more circular or radially elongated groups in the 

 European species. 



1 Weber and Sterzel (96) B. p. 102, fig. 26. 2 Seward (93). 



3 Renault (76) B. * Penhallow (97 



