VII. FORMULA EXPLAINED. 69 
as space may admit or records supply them. The ranges of High- 
land mountains, situate about the fifty-seventh line of latitude, 
and which culminate in Ben Nevis (1458 yards or 4874 feet) and 
Ben-na-muick-dhu (1440 yards or 4320 feet), have supplied most 
of the specified altitudes for North Britain. The most carefully 
made measurements have been taken on these mountains; and in 
general, owing to the greater elevation and extent of that mountain 
tract, it is the one best adapted for showing the heights attained 
by plants in this island. But doubtless many of the species 
could exist at a considerably higher elevation in the latitude of 
North Britain, if its hills were so much higher as to afford 
suitable situations for them above their actual limits. For most 
of the plants which fail to reach the uppermost zone, some indi- 
cations of altitudes attained in Mid Britain are added. These are 
mostly re-copied from a list given in the fourth volume of the 
Cybele Britannica, supplemented by the statements of altitude 
since printed in Mr. J. G. Baker's excellent work bearing the title 
of “North Yorkshire—Studies of its Botany, Geology, Climate 
and Physical Geography.” For many of the species which are 
found only in the three lower zones, the actual upper limits are un- 
ascertained, and can seldom be a matter of much interest. Unless 
known to rise above 150 yards from the sea-level, their altitudinal 
position may be sufficiently expressed in general terms. The 
words “ Low grounds” will thus be understood to express places 
of any height between the coast-level and 150 yards or nearly 500 
feet ; the words “Coast” or ‘‘ Coast-level” being used for plants 
known or supposed to grow considerably lower than 100 yards. 
* Littoral” plants are those of the seashore and its immediate 
vicinity ; mostly restricted thereto, as the Euphorbia Peplis and 
Atriplex arenaria; occasionally found inland likewise, at a low 
elevation, as the Asplenium marinum and Erodium maritimum, 
notwithstanding their sea-side names; and some of them rising to 
or reappearing high upon the mountains, as the Armeria maritima 
and Silene maritima. These terms are seldom introduced, how- 
ever, unless to intimate that no higher elevation has been ascer- 
tained ; although sometimes used with intent to show a descent to 
