17 

 KEY TO THE HARPIDIOID HYPNA. 



I. A. WHELDON. 



So much has been written upon this polymorphous group of 

 plants, that some apology is perhaps needed for this paper. 

 The key here presented was drawn up at the request of one 

 of my most stimulating bryological correspondents, the late 

 Prof. T. Barker. He wrote to me in March, 1902, criticising 

 my ' North of England Harpidia ' which had just appeared 

 in The Naturalist to the following effect :— 



' What I chiefly miss in the paper itself is a " Key " to the 

 species, varieties and forms. Without this it often takes a 

 long time to find the name of any specimen. With a key you 

 get quickly to the probable name, and then it does not take long 

 to compare the specimen and description, and see if they agree.' 

 Later he again wrote : ' I am strongly prejudiced in favour of 

 keys. I think they should be made even if not printed, because 

 the process, almost more than anything else, compels a man to 

 form clear crisp opinions. It is an article of my creed that 

 when a man who knows a group of plants or animals cannot 

 make a key that will work, it is because the species and varieties 

 are not distinct. . . . Schimper's " Synopsis " is to me a very 

 difficult book, and almost hopeless unless I can first otherwise 

 get at the probable name of a specimen.' In a later letter offer- 

 ing much useful and helpful criticism, he writes : ' I must not 

 keep your key to the Harpidia longer. It seems to me to be 

 really a key, and not a series of condensed descriptions. A 

 dichotomous key is a very great help to a tyro ; those in Dixon's 

 Handbook are very time-saving, and the want of them makes 

 it very tedious to find out an unknown moss by help of 

 Schimper's "Synopsis." I hope you will publish the key in 

 due course, as it is too useful to be consigned to oblivion.' 



After eighteen years, I find the key has needed many 

 alterations. Not the least difficulty has been the selection of 

 a generic name or names ! At first the choice was between 

 Amhlystegium, Hypnum and Drepanocladus. As the study of 

 mosses has progressed, there has been a gradual evolution of 

 our ideas as to the limits of genera. Such genera as Junger- 

 mannia, Hypnum, etc., served their purpose until, by the 

 growth in the number of species, they became too unwieldy. 

 They were then divided into sections, which were speedily 

 raised to sub-genera and finally to genera. The Harpidioid 

 Hypna were for a time classed under Amhlystegiuvi ov Hypnum, 

 under sections ' Harpidium,' ' Scorpidium ' and ' Cratoneuron.' 

 These groups next emerged as genera, the first named becoming 

 Drepanocladus, because Harpidium was untenable. In 

 common with other hypnoid genera, such as Campylium, 



1921 Jan. 1 



