Barber . — Ctipressinoxylon vectense . 355 
general separation of species, and yet there are certain genera 
which show marked and apparently constant peculiarities in 
one or other of these respects. A careful analysis is therefore 
necessary of the medullary rays and their cells. Such has 
been attempted in our sections, following as far as possible 
the lines laid down by Kraus, Essner, and Beust in their 
investigation of living forms. 
At the instigation of Kraus, Essner has shown that the 
numbers of medullary rays per sq. mm. of tangential section 
in any species is greatest in the first year, and that it 
decreases gradually outwards, while on the other hand the 
medullary rays are lowest near the pith and increase in 
height towards the cortex. This has led him to the idea 
that the number of medullary cells per sq. mm. may be 
constant for the same species in different years. In his 
attempts to obtain characteristic numbers for different species, 
he found, however, that individuals of the same species varied 
more than did those of different species. This character 
cannot therefore be considered absolute. 
Beust, arguing from the intrinsic importance of the 
medullary-ray-tissue in the life of the wood, regards it as 
probable that some relation exists between the relative 
bulk of this tissue and that of the tracheides. To obtain 
this proportion he has combined the several factors deter- 
mined by Essner. It is however not certain at present 
how far the proportion will hold good for the different types 
of wood. 
I have attempted to apply these observations to the fossil 
sections. It is not easy to determine accurately the age 
of the part in a tangential section of a fossil. In my 
specimens they have been cut as a rule between the sixth 
and the tenth rings of growth. The following figures repre- 
sent a great deal of labour. Their worth will not be fully 
apparent until it is possible to compare them with those 
of other specimens. As their value greatly depends upon 
the number of medullary rays observed, I have where possible 
noted this. 
