94 LINKS WITH THE PAST [ch. v 



recently discovered in Borneo is more closely con- 

 nected witli the Assam type than with those of the 

 IMalay region. Until a few years ago the genus 

 Dipteris was included in the large family Polypodi- 

 aceae of which nearly all our British ferns are 

 members, but the discovery of certain distinguishing 

 features in the structure of the sporangia showed 

 that these Eastern and Southern species form a 

 fairly well-defined group worthy of family rank. 



In the Rhaetic plant-beds of Northern and 

 Central Europe, of North America, Tonkin, and 

 elsewhere, numerous fossil leaves have been dis- 

 covered which in shape, venation, and in the 

 manner of occurrence of the sporangia bear a close 

 resemblance to species of Dipteris. Ferns of this 

 type were abundant in the Jurassic floras of the 

 northern hemisphere, and it is interesting to find 

 impressions of Dipteris-like leaves both in the 

 Jurassic rocks of the Yorkshire coast as well as in 

 slightly newer beds of the same geological period on 

 the north-east coast of Sutherland. 



It is impossible to say with confidence how nearly 

 these Rhaetic and Jurassic ferns were related to the 

 existing species, as our knowledge of them is less 

 complete than in the case of the fossil representatives 

 of the Matonineae, but there can be no reasonable 

 doubt that in Dipteris as in Matonia we have a con- 

 necting link between the present and a remote past. 



