192 
XIII. PTEKIS. 
out at intervals the almost opposite bipinnate pinna, 
several pairs being in many cases produced. The form of 
the primary pinnae is usually ovate, that of the secondary 
pinnae narrow-lanceolate, the former being nearly opposite 
in distant pairs, and the latter near together, either oppo- 
site or alternate along the secondary rachis. The pinnules 
are attached by their base without a footstalk either al- 
ternately or opposite along the tertiary rachis, becoming 
confluent towards the apex, the terminal often consider- 
ably larger and more elongate than the rest. Their form 
is oblong-lanceolate, deeply pinnatifid, or sometimes only 
sinuate, the lobes being oblong-obtuse. The venation is 
arranged thus : the mid-vein produces a series of veins, 
which become mid-veins to 
the lobes, and these branch 
into a further series of 
venules, which are either 
once or twice forked, and 
extend to the margin ; the 
lowest branch right and 
left of the mid-veins of the 
lobes usually meet and 
unite, forming a rather 
irregular longitudinal vein 
parallel with the mid -vein 
of the pinnule ; and along 
the edge of the pinnules, 
and also of the lobes and interstices, extends a marginal 
vein, to which the ends of the other veins are united. 
This marginal vein bears the fructification, the spore-cases 
being arranged on it in a continuous line, and covered by 
a bleached membranous reflexed extension of the epidermis 
of the upper surface of the frond, which serves as an in- 
dusium ; the epidermis of the under surface being also 
developed in a similar way, and forming a membrane on 
which the spore-cases lie; both membranes being frined 
at the margin with jointed hairs. The fructification is 
