BRITISH SPECIES OF THE GENUS MNIUM. 37 
added: their sum is the figure of the leaf. Places immediately below a 
tooth are excluded (fig. 6, n). 
Example: In fig. 6, I number 3 cells 
at a, 4 cells at b, 4 cells at c (in- 
cluding ¢), 2 cells at d. I take the 
maximum: 4 cells. 
There is no difficulty if the border 
contains one layer of cells (fig. 6). If 
the border-cells are placed in two (or 
more ? ) superposed layers, the cells are 
numbered as they are seen in the optical 
section: here an error of one cell is 
possible, and this may occur in the 
same sense at both sides of the leaf. 
I estimate the possible positive or 
negative error of the figure of the 
leaf at 2 cells (1 cell if the cells are 
not numerous) *. 
$31. BREADTH OF THE BORDER.— 
This character also is very variable along 
the margin. I measure the breadth at 
three places (same method as for the 
number of cells, § 30) and take the 
mean value of the three measurements. 
The sum of the two mean values (right 
and left border of the leaf) is the figure 
of the leaf. I always exclude the in- 
terior border-cell, but I include the 
exterior wall of this cell in the measure- 
ment.— Example (fig. 6): Measuring 
Fia. 6.—Border of a leaf—B, place of 
the breadth of the border at a, I take the greatest breadth of the leaf. 
the two exterior cells, including the t, tooth ; a, b, c, d, e, n, see § 30. 
wall between the 2nd and the 3rd cell ; 
measuring at B I take three cells, excluding e and including the exterior 
wallof e; measuring at d I take the exterior cell and the interior wall of 
this. I calculate the mean value of the measurements a, b, c, and add this 
to the mean value of the other side of the leaf. I have used obj. 6 Leitz. 
Iestimate the possible positive or negative error of the figure of a leaf 
at 5 p. 
* If the border is recurved, it is, of course, impossible to count the border-cells without 
making sections 
