THE LIMESTONE POLYPODY 
267 
leaves. The pinnae are not three-branched, and the prin- 
cipal rachis in continuation of the stipes upwards is stouter 
than the rachides of the largest pair of pinnae — the 
lowest on the frond. In Dryopteris the rachis of each 
branch is of the same thickness, and this, taken in connexion 
with the greater proportionate development, and the more 
distinct character of the lowest pair of pinnae, gives the 
decided three-branched appearance to that species. In Cai- 
rn rmm the fronds may be described as being bi-pinnate — - 
in large specimens, tri-pinnate in their lowest pinnae. The 
lowest pair of pinnae set on nearly at right angles to the 
principal rachis are narrowly triangular, pinnate— some- 
times bi-pinnate at their bases — pinnatifid towards their 
apices, the lower pinnules 011 their mid-stems being longer 
than the upper ones, and again divided, or deeply cleft, at 
their bases, into lobes, but merely notched towards their 
apices. The pair of pinnae next above the lowest pair are 
more narrowly triangular, pinnate at their bases, pinnatifid 
higher up ; the pinnules — where pinnate— attached to the 
mid-stems of the pinnae sometimes by very short stalks, 
and having their margins serrated, the lower pinnules being 
only slightly longer than the upper ones. The next and 
succeeding pairs of pinnae become narrower and narrower, 
and less and less divided, until they finally run together 
at the ajjex of the frond. The venation is almost iden- 
tical with that in Dryopteris, and consists of a wavy mid- 
vein, with alternate, sometimes simple and sometimes forked, 
venules, bearing small uncovered or non-indusiate heaps 
of spore cases, which are evenly distributed over the under 
side of the pinnules, or lobes ; sometimes becoming almost 
confluent along the lobe margins, and then giving to the 
fronds a miniature resemblance to the Bracken when its 
fructification is ripe. This resemblance is made the greater 
from the fact that the lobe margins are a little curled 
