SPECIES AND VARIETIES OF BRITISH FERNS. 21 



and surrounded ly urn-shaped or two-valved mem- 

 branes. 



15. Tnc/iOTOa/ies:=Marginal-fruited Ferns, having the sori 



surrounded by urn-shaped expansions of the frond. 



16. IfymenophyUum=NaTginal-fmited Ferns, having the 



sori surrounded by two-valved expansions of the 

 frond. 



IL OSMUND ACE^E. This group consists of Ferns having 

 the young leaves circinate, the spore-cases destitute of 

 an elastic ring, and bursting vertically by two regular 

 valves. 



17. 0s#iM^a=Marginal-fraited Ferns, having the regular 



valved spore-cases in irregular, dense, branching 



clusters, terminating the fronds. 



III. OPHIOGLpSSACELE. This group is composed of 

 Ferns having the young leaves folded up straight, 

 the spore-cases destitute of an elastic ring, and two- 

 valved. 



18. .5ctfrycAttm=Marginal-fruited Ferns, having the spore- 



cases in irregularly branched clusters, on a separate 

 branch of the frond. 



19. C^Aio^/fosswm^Marginal-fruited Ferns, having the spore- 



cases sessile in two-ranked simple spikes terminating 

 a separate branch of the frond. 



Genus 1. POLYPODIUM, Linnceus. 



1. P. mdgore. Fronds oblong pinnatifid. 



Var. semttacerum. Fronds pinnatifid, lower segments 



again pinnatifid, upper fertile. 

 Var. cambricum. Fronds pinnatifid, segments again 



pinnatifid, all barren. 



2. P. Phegopteris. Fronds pinnate below; pinnae pin- 



natifid. 



3. P. Dryopteris. Fronds ternate, deltoid, glabrous. 



4. P. Robertianum. Fronds subternate, elongate, deltoid, 



glandular-mealy. 



5. P. alpestre. Fronds bipinnate, lanceolate. 



Var.Jiexile. Fronds narrow, flaccid ; pinna deflexed. 



Genus 2. ALLOSORUS, Bernhardi. 

 1. A. crispus. The only British species. 



Genus 3. GYMNOGRAMMA, Desvaux. 

 1. G. leptopliylla. The only British species. 



