70 HISTORY OF BRITISH TERNS. 



each pinna having the margin divided by four or five deep, 

 sharp teeth, the rest of the pinnules and lobes having from 

 one to three similar teeth. The main rachis of the frond, 

 as well as the partial rachis of each pinna, have a narrow 

 winged margin, that is to say, a very narrow leafy expansion 

 along their sides, throughout their length ; and this is per- 

 haps the most obvious technical point, except size, by which 

 to distinguish the present plant from A. lanceolatum. In 

 structural details they very much resemble each other, so 

 that in description they appear very similar, although to the 

 eye they are at all times distinct. 



The fronds being rigid and opake, the venation is less 

 evident than is usual in Ferns. It consists, in each pinnule, 

 of a central or principal vein, which throws off a venule 

 towards each lobe or serrature, and in the larger pinnules 

 some of these venules become divided, so that a veinlet is 

 directed towards each of the serratures into which the mar- 

 gin is divided. On two or more of these veins a sorus is 

 produced, which in form is short compared with those pro- 

 duced by most of the genus ; the actual form is oblong, 

 rather flat on the side by which they are attached; and they 

 are covered by an indusium of similar form, which is waved 

 and indented on the free margin. Sometimes the sori keep 



