PRIMARY STRUCTURE OF CERTAIN PALAEOZOIC STEMS. 357 



Taking, with free modification, the more essential parts of Goeppert's generic 

 diagnosis,* and adding the newly discovered primary characters, we may define the 

 ffenus as follows : — 



Pitys, Witham, emend. 



Trunks arboreal, with wood resembling that of recent Araucariese. 



Trunks composed of a large central medulla, and a zone of wood, in which the 

 presence of concentric layers is inconstant. 



Medulla surrounded by numerous small strands of primary xylem, which pass out 

 through the wood. Tracheides with bordered pits ; pits in three or four series, on the 

 radial walls, spirally arranged, usually contiguous and hexagonal ; sometimes occurring 

 on the tangential walls also. Larger medullary rays formed of 2, 3, 4, or more series 

 of cells. 



The old genus Pitys, rightly revived by Goeppert, turns out to be quite neatly 

 characterised by its numerous small mesarch strands of primary xylem, as well as by 

 the large pith, and multiseriate rays, which have hitherto served to distinguish it. 



III. Dadoxylon Spenceri, sp. nov. 



This stem was found by the late Mr James Spencer in the Horse Bridge Clough 

 at Hebden Bridge, about six miles to the west of Halifax.t I am only acquainted with 

 one specimen of importance. A second specimen, from the same locality, appears to 

 have a similar structure, but the preservation is too imperfect for determination. 



Sections of the good specimen are preserved in the Williamson Collection, and also 

 in Mr Spencer's private collection, which, since his death, has come into my possession. 

 The horizon is given by Mr Spencer as that of the Yoredale Kocks, while Williamson, 

 who described the specimen in 1879, speaks of it as derived from "the Marine Ganister 

 Bed near Halifax." J This apparent contradiction, involving a doubt whether the 

 fossil belonged to the Lower or Upper Carboniferous, led me to make inquiries of my 

 friend Mr Kidston, who referred the question to Dr E. D. Wellburn, of Sowerby 

 Bridge, who has made a special study of the Geology of the district. Dr Wellburn's 

 conclusion is that the rocks at Horse Bridge Clough are not the equivalents of the 

 Yoredales of Phillips, but are a newer group of rocks belonging to the Upper Carbon- 

 iferous division. It thus appears that the true horizon of the specimen cannot be very 

 different from that assigned to it by Williamson ; Dadoxylon Spenceri is consequently 

 the only stem of Upper Carboniferous age dealt with in the present paper. 



Williamson, in his description of the specimen, calls attention especially to the 



* L.c, p. 403. 



t The discovery of the prolific plant-bed in which the Dadoxylon occurred is recorded by Mr Spencer id his 

 paper on "The Yoredale and Millstone Grit Rocks," published posthumously in the Proc. Yorkshire Gaol, and 

 Polytechnic Soc, New Series, vol. 13, 1898 (see p. 378). 



X " Organization of Fossil Plants of Coal-Measures," Part X., Phil, Trans., 1880, pt. ii. p. 516, PI. 20, fig. 60. 



