BRITISH SPECIES OF THE GENUS MNIUM. 39 
it is often practically impossible to determine the distal limit of the nerve. 
Therefore I admit three degrees: 1*, the midrib is short and does not reach 
the summit (fig. 8, 7) ; 2°, there is a trace of relation between the extremity 
of the nerve and the summit (fig. 8, 2) ; 3°, the nerve undoubtedly reaches 
the summit (fig. 8, 3). 
There is no hesitation possible about 1° and 3°. In the 2nd form (inter- 
mediate between 1? and 3°) it is sometimes difficult to come to a decision. 
I bring under 1° (nerve ceasing below the summit; abbreviation —) a leaf in 
which there is at least one ordinary (isodiametric) cell above the extremity 
of the nerve (in fig. 8, 7, there are two such cells). 
(vL a 
Fic. 8.—Relation between the nerve and the summit of the leaf. n, nerve. 
See text. 
About the summit of the leaf the two differentiated borders meet each 
other, and we find a greater or smaller number of differentiated cells. If 
the nerve reaches those cells, I bring the leaf under 3° (abbreviation +. 
Fig. 8, 3: the nerve often reaches further than in this figure). If the 
extremity of the nerve is connected with those differentiated cells by one or 
more distinetly elongated cells, which constitute a sort of bridge among the 
ordinary isodiametric cells, I bring the leaf under 2° (fig. 8, 2. Trace of 
relation; abbreviation +). 
In some eases the decision is rather arbitrary, because the structure near 
the summit is sometimes indistinct. 
$36. ToorH AT THE SUMMIT OF THE LEAF.—No explanation is needed 
about this character. (Tooth existing, abbreviation + ; tooth not existing, 
abbreviation —.) 
§ 37. NUMBER or Fruits.—See the explanation in Table XX XVIII. 
