those which form the rest of the outer zone (fig. 461, B, _b_); (ii) a zone of tracheal tissue continuous with and originally identical in appearance—except that the elements are rather narrower—with the outer secondary xylem; (iii) an excentrically situated island composed of tracheids enclosing a small central area occupied by thin-walled parenchyma. This third region, represented by black patches in fig. 461, A, in all probability represents the primary part of each stele to which the rest of the tissue has been added by the cambium. A striking feature of the secondary xylem is the absence of medullary rays: the tracheids resemble those of _Cladoxylon_ and _Protopitys_ in the transverse elongation of the pits (fig. 461, D) which form either a single row or several irregularly distributed rows. The primary xylem consists in the peripheral region of tracheids with very narrow scalariform pitting which at first sight suggest close spiral bands (fig. 461, C), while the inner tracheids are either annular or reticulate and associated with elongated parenchyma. The imperfectly preserved ground-tissue appears to consist of homogeneous parenchyma with radially disposed bands of stereome in the outer cortex. [Illustration: Fig. 461. _Völkelia refracta._ A. Transverse section of a specimen in the Breslau Museum. B. Portion of a stele; _a_, the crushed xylem on the outer side; _b_, the inner side. C. Longitudinal section showing the median protoxylem. D. Tracheid from the secondary xylem. (After Solms-Laubach.)] Our knowledge of _Völkelia_, though far from complete, justifies its generic separation from _Cladoxylon_ from which it differs in the lack of medullary rays and in the structure of the primary portion of each stele. In the form and arrangement of the pits in the secondary tracheids _Völkelia_ differs from _Medullosa_ and resembles _Cladoxylon_. The opinion expressed by P. Bertrand[529] that _Völkelia_ is probably the stem of one of the Zygopterideae is based on the older accounts of the genus and not on the fuller description of 1910. =PROTOPITYEAE.= =Protopitys.= Goeppert. The only species so far described is that for which Goeppert founded the genus in 1850, substituting _Protopitys_[530] for the name _Araucarites_, adopted in an earlier paper, on the ground that the structure of the xylem denoted a distinct generic type. The type-species is from the Upper Devonian rocks of Falkenberg in Silesia. _Protopitys Buchiana_ Goeppert. 1845. _Araucarites Buchianus_ Goeppert, in Wimmer’s Flor. Schlesien (edit. +ii.+) p. 218. 1847. _Pinites Goepperti_ Unger, Chlor. Protog. p. 31. 1847. _Dadoxylon Buchianum_ Endlicher, Syn. Conif. p. 300. 1850. _Protopitys Buchiana_ Goeppert, Foss. Conif. p. 229, Pl. +xxxvii.+ figs. 4–7; Pl. +xxxviii.+ figs. 1, 2.