100 



MISS KELLIE BANCROFT ON 



in that it provides a parallel case to that put forward by Miss de Praine (l) for the 

 Cycadacese, which she considers may have originated from a monostelic Medullosean 

 form typified by Sutcliffia *, which shows a more Cycadean type of pitting. 



The age of the South African specimen, as has been shoAvn, is doubtful, but its structure 

 justifies its reference to the Palaeozoic era, while its agreement in general anatomy with 

 the Continental MeduUoseae rather than with the earlier English types is consistent 

 with reference to an Upper Permian horizon. 



Prom early Palaeozoic to Permian times there seems to have been a plexus of types 

 showing polystelic structure, and the South African stem may be included as a member 

 of this plexus. To recapitulate its probable afiinities : — The stem seems to agree most 

 closely with the genus Ifeclullosa, which is recognised as showing considerable variation 

 of stelar structure. It differs from the typical species f, however, in the extreme 

 reduction of the partial pith, in the general coniferous texture of the xylem with its 

 compact structure and uniseriate rays, as well as in the pitting of the elements, which 

 in this respect resemble those of the Cordaiteae or the Araucarian conifers. Steloxylon 

 is approached chiefly in the type of pitting of the tracheides, but it differs in the less 

 definite arrangement of the steles and in the presence of a partial pith — this being much 

 reduced in the South African stem. 



These differences, united to the facts that the horizon and locality of the stem- 

 fragment are indefinite and that its associations are indeterminable, sufficiently isolate 

 it from all hitherto- described types to render its reference to any one of them undesirable. 

 Eurther, this fossil appears to be the first, and so far the only, specimen of a stem 

 of this nature known from the Southern Hemisphere, so that it seems justifiable to 

 institute a provisional genus for its reception and to include the genus in the family 

 Medullosese. 



The generic name MJiexoxylon % is suggested as being descriptive of the broken-up 

 natme of the vascular structures of the stem. 



IV. SrMMARY AND DIAGNOSIS. 



(1) Rhexoxylon africamim, a stem-fragment of doubtful lat^r Palseozoic age, from 

 sinknown South African locality, is described and diagnosed as follows : 



Ehexoxylon, nov. gen. 

 Vascular system of the stem consisting of an inner ring of elliptical steles, each com- 



posed of a large inversely orientated part, from which traces are given off, and a small 



normal part ; external to these a series of " partial steles " consisting only of normally 

 orientated elements ; the lateral margins of these partial steles often much broken by 

 the separation of traces. Xylem compact, having spiral elements on the inner margin ; 



Of. Worsdell's views on tl 

 t M. Solmsii must be escep 

 + Gr., pTiyvv/jii. — I break up. 



Note remarks on p. 96. 



