54 Dr. W. C. Williamson on 



tion. I judged it wiser, therefore, to issue from time to 

 time, memoirs which should contain whatever definite facts 

 I succeeded in discovering about any of the Palaeozoic types 

 of vegetation ; filling up the gaps in the record whenever 

 further researches threw additional light on any of these 

 types. But this method is also attended by some serious 

 drawbacks. Even when published in successive volumes of 

 the same journal, as in the case of the seventeen memoirs 

 that have appeared in the Philosophical Transactions from 

 1 87 1 to the present year 1890, the labour of hunting 

 through numerous big volumes, and then piecing together 

 my detached observations on each special plant, has become 

 very serious. The difficulty becomes still more serious, 

 when such memoirs have been published in different jour- 

 nals. Yet the number of new facts which I have recorded 

 makes it indispensable that Palaeo-botanists, who are my 

 contemporaries, or who may be my successors in similar 

 labours, should have those facts put as easily as possible 

 within their reach. Living Palaeontologists have already 

 felt the difficulties to which I have referred ; I have received 

 many communications, both from my fellow-countrymen 

 and from foreign correspondents, expressing the wish that 

 I would prepare a collective index to the entire series of 

 my Palaeo-botanical publications. Having recognised the 

 reasonableness of the demand, I now commence my response 

 to it. 



The Council of the Literary and Philosophical Society 

 of Manchester having kindly invited me to prepare a series 

 of such indexes for publication in their Memoirs and Pro- 

 ceedingSy I lay before the Society the first part of such a series. 

 Each succeeding part will embrace one or more of the great 

 families of Palaeozoic plants that have been the subjects of 

 my researches. The descriptions and figures of each special 

 organ of such plants will, as in my present contribution, 

 be classified morphologically and histologically. Hence the 



