1024 
Scott . — The Structure of 
to Mesoxylon , under Cordaiteae (1. c., p. 526). The centripetal wood of the 
stem, no doubt, died out gradually ; in Mesoxylon it is still sufficiently well 
developed to justify generic separation from Cordaites , but not, I think, 
a position in a distinct family. The point, however, is of little importance ; 
what really matters is, that the two families Cordaiteae and Poroxylae are 
now closely linked together. 
The points of resemblance and difference between Mesoxylon , Cor - 
duties, and Poroxylon have been well summarized by Mr. Maslen (1. c., 
p. 410). They hold good essentially for all the species investigated. 
It will be noticed that no definite distinction between Mesoxylon and 
Cordaites is given, beyond the presence of centripetal xylem in the stele of 
the former. In other respects there appears to be agreement, but we 
do not know enough about the exact course of the leaf-traces or the 
detailed structure of the phloem in Cordaites , to be certain how close 
the agreement really was. At any rate, we have the large, discoid pith, 
the dense wood with narrow rays, 1 and the histology of the xylem-elements, 
especially the great development of spiral and scalariform tracheides in the 
leaf-trace region, as definite characters common to the two genera. The 
species of Mesoxylon would have been left in Cordaites if it were not for the 
presence of centripetal wood in the stele. I feel no doubt that most of the 
British specimens of ‘ Cordaitean 5 leaves really belong to Mesoxylon , which 
is a much commoner type of stem in the Coal-Measure petrifactions than 
that of Cordaites itself. If this belief is confirmed, we shall have the close 
agreement in foliar characters as another proof of the near affinity of 
the two genera. 
I thus regard Mesoxylon as the last term in the series of forms leading 
up from the Seed-Ferns to the typical Cordaites described by Grand’Eury 
and Renault. 
Some additions have lately been made to the Cordaitales and their 
allies by Dr. Zalessky, who has founded no less than five new genera, 
partly for new forms, partly for plants already described under other names 
(Zalessky, ’09, Tl 1 , and ’ll 2 ). 
The new forms are : Callixylon Trifilievi , Zal. (Zalessky, ’09 and ’ll 1 ), 
and Caenoxylon Scotti , Zal. (Zalessky, ’ll 2 ). The other new genera are : 
Eristophyton (Zalessky, ll 1 ), based on Calamopitys Beinertiana (Gopp.),and 
C. fascicularis , Scott ; Mesopitys (Zalessky, ’ll 1 ), based on Dadoxylon Tchi- 
hatcheffi (Gopp.); and Parapitys (Zalessky, Tl 1 ), based on Dadoxylon Spenceri , 
Scott. 
Callixylon Trifilievi (Zalessky, ’09) from the Upper Devonian of the 
Donetz basin in Russia is a very interesting fossil, evidently allied to the 
Lower Carboniferous genus Pitys (Scott, ’02). Numerous (2 6) small, mesarch 
1 As a matter of fact somewhat wicbr rays occur in Cordaites than have yet been observed in 
Mesoxylon (Renault, ’96, p. 334 ). 
