X 
DEFINITIONS. 
rough ( muricate ), or bear hard spines {aculeate). The fertile 
frond, or a portion of it, is frequently contracted, and differs en- 
tirely in aspect from the sterile. 
The veins { venation ) are produced from both sides of a central 
midrib {costa) , or from one side of an excentric or unilateral 
midrib, or from the base of the frond or segment, the midrib 
being obsolete. The first or 'primary veins form a more or less 
acute angle with the costa, having an oblique, upward direction 
towards the margin. They are either simple , or once or more 
times dichotomously branched (forked), or they are costceform, 
and produce branches on each side (pinnate), the branches (ve- 
nules) being simple or again forked, and terminate at or within 
the margin (free), or the apices of the proximate venules of each 
fascicle unite in an angular or arcuate manner, forming areoles 
(anastomosing). From the angular junctions or sides of the ve- 
nules, one or more outwardly directed tertiary veins (veinlets) are 
produced, and are either free and terminate within the areoles, or 
anastomose in the angle or sides of the next superior pair of ve- 
nules ; or the venules are unequal, and irregularly anastomose, 
with variously directed, free or conniving veinlets, terminating 
in the areoles ( compound anastomosing). When the primary 
veins are obsolete or evanescent, the venules and veinlets are ge- 
nerally uniform, and all united (reticulated). 
All the above-described varieties of venation are more or less 
distinct, plane, or elevated (external), or immersed in the sub- 
stance of the frond, and then often indistinct {internal). 
The sporangiferous receptacle is a thickened point or length- 
ened portion of the ultimate venules or veinlets, and is either 
formed on their apex {terminal), or between their apex and base 
(medial), or on the point of forking (axillary), or on the angular 
crossings or points of confluence of two or many venules or vein- 
lets (compital), or the whole or a portion of the disc of the frond 
