1028 
Scott. — The Structure of 
Alethopteris). Whether Calamopity s fascicularis and Beinertiana attained 
a very large diameter or not we cannot say ; if they did so, the fact would 
certainly be no argument against their Pteridospermous affinities. 
Summary. 
The characters of Mesoxylon Lomaxii and M.poroxyloides have already 
been summarized in their specific diagnoses (pp. 1016 and 1023). The 
genus comes very near Cordaites , as shown by the characters of the pith 
and wood, and further indicated by those of the associated leaves. The 
affinity with Poroxylon is somewhat more remote, and the genus is best 
placed in the family Cordaiteae ; it is at present definitely distinguished 
from Cordaites only by the presence of centripetal wood in the stele of the 
stem. Mesoxylon thus forms the last link in the chain of fossil types 
connecting the Pteridosperms with the typical Cordaites of the Upper 
Palaeozoic. 
Of the new genera recently established by Dr. Zalessky, Callixylon 
is very near Pitys and should be placed in the same family. Caenoxylon 
and Mesopiiys are advanced forms, and had reached an anatomical level 
corresponding to that of Cordaites itself, but on somewhat different lines. 
Parapitys is best regarded as a near ally of Mesoxylon. Eristophyton, 
even if generically separable from Calamopitys, is closely allied to it, and 
both may provisionally be regarded as probably belonging to the Pterido- 
sperms rather than to the Cordaitales. It is possible, however, that 
further discoveries may show that the Calamopityeae were an important 
transitional group. 
The results already attained by Dr. Zalessky and others are very 
satisfactory, as demonstrating a considerable variety among the stems of 
Palaeozoic age, referable or allied to the Cordaitales. 
References. 
Benson (’12) : Margaret Benson, Cordaites Felicis , sp. nov., a Cordaitean leaf from the Lower Coal 
Measures of England. Ann. of Bot., vol. xxvi, 1912, p. 201. 
Maslen (’ll) : A. J. Maslen, The Structure of Mesoxylon Sutcliffii (Scott). Ann. of Bot., vol. xxv, 
19m, p. 381. 
Renault (’79) : Bernard Renault, Structure comparee de quelques tiges de la Flore Carbonifere. 
Nouvelles Archives du Museum, ii, 2 me serie, pp. 270-8. Paris, 1879. 
(’96) : Bassin houiller et permien d’Autun et d’Epinac. Flore Fossile. Etude des Gites 
mineraux de la France, pp. 332-52. Paris, 1896. 
Scott (’02) : D. H. Scott, On the Primary Structure of certain Palaeozoic Stems with the 
Dadoxylon Type of Wood. Trans. Royal Soc. Edinburgh, vol. xl, Part II, 1902, p. 331. 
(’09) : Studies in Fossil Botany. Second Edition, Chapter xii, 1909. 
and Maslen (TO) : D. H. Scott and A. J. Maslen, On Mesoxylon , a New Genus of Cordai- 
tales. Preliminary Note. Ann. of Bot., vol. xxiv, 1910, p. 236. 
