INTRODUCTION TO CRYPTOGAMIO BOTANY. 387 



instances of the wide diffusion of the jDnncipal genera. In 

 truth, it is very unsafe to prescribe absolute limits to the diffu- 

 sion of the genera. Eecent discoveries more and more tend to 

 show that they have a far wider range than has generally been 

 supposed. This remark will not, however, be made without 

 attempting to prove it. The genus Chiodecton, first described 

 by Acharius, in ] 815, was supposed at first to be wholly tropi- 

 cal ; but a species was found in 1824 in the lies d'Hy feres, 

 and another more recently in Ireland by the late Dr. Taylor. 

 The genus Olyphis, which was described by Acharius at the 

 same time, from tropical species, is absent from the Licheno- 

 graphia Europjea of Fries (1831) ; but a species has been col- 

 lected in Portugal by Dr. Welwitsch, and in Ireland by Sir 

 Thomas Gage. The Glyphidei contain a fair number of 

 species (some of which are common) in tropical America, and 

 eleven have been collected by M. Leprieur, in the neighbour- 

 hood of Cayenne alone. Again, if there be one feature pecu- 

 liar to tropical Lichenology above another, it is the frequency 

 of the strictly epiphyllous species, more especially of the genus 

 Strigula. Yet Fries first included one (seemingly imperfect) 

 species in his Lichenographia Europsea, found in Switzerland ; 

 a.nd another was found in 1848 in this country, where it 

 appears to be very common. An epiphyllous Parmelia has 

 also been lately found in England, which is probably only a 

 wanderer from its true tropical home. These three genera are 

 still wanting in temperate and arctic North America. Con- 

 versely, it is certain that some genera which had only been 

 found in the northern hemisphere, occur towards the extremity 

 of the southern. If any one should be asked to select a genus 

 abounding in Europe and the more northern America, but not 

 occurring elsewhere, he would no doubt fix upon Calicium ; 

 but Mr. Colenso has lately gathered a species in New Zealand. 

 Again, if any one large genus of Lichens is pre-eminently tro- 

 pical, it is the genus Trypethelium. Fifteen Trypetheliei 

 have been found near Cayenne alone. Not a species has yet 

 been discovered in Europe or North (temperate and arctic) 

 America; and we had never seen a weU-marked species of the 

 genus from any country south of the tropics, till Mr. Colenso 

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