THE SHIELD FERN. 115 
us the only alternatives are to receive three species, or one only; 
and we do not find this latter course anywhere maintained. In 
the ease of British specimens there is little difficulty, if any, in 
distinguishing three species. It is only when exotie forms are 
also taken into account, that serious difficulty arises. Even in that 
case, however, we rather prefer to consider this as one of those 
numerous instances in which specific limit is not clearly definable, 
than as an example of specific identity under phases of extreme 
diversity of character. Indeed, if such comprehensive species are 
admitted, it will become utterly impossible to define them. 
The majority of the species of Polystichum, including all those of 
British origin, belong to the $ Hypopeltis, which may be considered 
as the typical group, distinguished by having its pinne and pin- 
nules continuous with the rachis. The $ Oyclopeltis, which we also 
include in the genus, and which is represented by the West Indian 
Aspidium semicordatum of Swartz, differs only in having the pinne 
articulated with the rachis. 
Beyond the foregoing, there is a group of species, represented by 
the old Aspidium aristatum, in which the indusium, which is roundish 
in outline, has only a shallow lateral notch, from which a furrow or 
depression extends inwards towards the centre; being attached at 
this part, and consequently towards the centre, many botanists have 
regarded the indusium as being peltate, and referred the plants to 
the genus Polystichum. In this view we do not agree. In reality, 
the indusia in the plants now referred to, differ only in degree 
from those of the Common Male Fern, being obviously roundish- 
reniform, so that these species, though polystichoid in habit, must 
be placed under Lastrea. 
The English name of Shield Fern was that given to the old 
genus Aspidium, and we therefore adopt it for the present group on 
account of its being the most typical of the two genera under which 
the English species of Aspidiwm are placed. The peltate involucre 
sufficiently justifies this adoption. 
The generic name is derived from the Greek polys, many, and 
stichos, order. 

