82 
HISTORY OF BRITISH FERNS. 
the receptacle than a proper investing membrane. A similar 
membrane has been observed in the species. The sori, 
both in P. alpestre itself, and in the variety flexile^ are not 
in all cases strictly punctiform, but occasionally, though 
rarely, lateral but hardly elongated on the veins ; and this 
is perhaps what induced Mr. Newman to propose the 
genus Pseudathyrium. We, however, agree with those 
who retain the plant in the Polypody family, and regard 
flexile rather as a variety than as a species, though it is 
certainly a remarkable one. This very interesting plant 
was found in Glen Prosen, in 1852, by Mr. Backhouse 
and Mr. Westcombe ; and the same botanists again found 
it plentifully in the same district of the Clova Mountains, 
in the summer of 1855. 
Genus IL ALLOSOBUS, or ROCK BRAKES. 
Of this family we have but one British species, the 
Allosorus crispus. It is known from all its fellow-country 
ferns by the coincidence of the following features. It bears 
fronds of two kinds, one being leafy and barren, e. with- 
out sori, the other contracted, and bearing sori ; and hence 
