J. SMITH ON THE GENERA. OF FERNS. 43 



margin (of the frond or segment) (juarginal), or between the 

 margin and midrib (intrmnarginal), or close to the midrib 

 {costal) ; when elongated they are either oblong or linear, con- 

 stituting a continuous or interrupted line, which forms more 

 or less of an angle with the midrib (oblique), or runs parallel 

 with the midrib or margin (transverse). They are either 

 naked, or each sorus is furnished with a membranaceous 

 covering of various forms called indusium, which rises from 

 the receptacle. 



The indushim is a plane, or vaulted, or cup-shaped mem- 

 brane, produced from the receptacle of each sorus (special), 

 and is generally deciduous as it becomes replicate. It is 

 either produced from the centre of the receptacle (central), or 

 on one side (lateral), or its base is attached all round the base 

 or margin of the receptacle (calyciforni). When central it is 

 usually orbicular, with its margin depressed and free all round, 

 or sometimes it is inflated. When lateral it is either reniform, 

 oval, or oblong, and attached by a point or depressed sinus of 

 its side ; or linear and attached its whole length on the side 

 of an elongated receptacle ; the other margin free. When 

 calyciforra, it is first entire and globose, at length opening with 

 an entire or laciniated margin, forming a cup which contains 

 the sporangia. Often the entire margin or lobules of the 

 frond is changed in texture, and forms an accessory indusium, 

 with which the interior lateral attached special indusium more 

 or less connives, and by their union they form a vertical or 

 reflexed, continuous or urceolate, calyciform or bilabiate, mar- 

 ginal cyst, which contains the sporangia, and is open ex- 

 teriorly. 



Sometimes the whole of the sori of each segment are in- 

 cluded within a universal indusium, which is formed by the 

 revolute margin of fertile contracted fronds. 



Obs. I. In the definitions of the venation and sori, it must 

 be understood that the whole is derived from a combined view 

 of the structure on one side of the ultimate midrib, unless 

 otherwise specified. 



Obs. II. The intention of this paper being merely aa 



