Palaeontoiogie. 471 



confusion is caused by the use of different generic and specific 

 names for plants that are in all probability identical, or at least 

 very closely allied. Worthless fossils are frequently designated 

 by a generic and specific title: an author lightly selects a nevv 

 name for a miserable fragment of a fossil fern-frond without 

 pausing to consider whether bis record is worthy of acceptance 

 at the hands of the botanical palaeographer. In endeavouring 

 to take a comprehensive survey of the records of plant-life, we 

 should aim at a wider view of the limits of species and look 

 for evidence of close relationship rather than for slight diffe- 

 rences, which might justify the adoption of a distifictive name. 

 Oiir object, in short, is not only to reduce to a common lan- 

 guage the diverse designations founded on personal idiosyn- 

 crasies, but to group closely allied forms under one central 

 type. We must boldly class together plants that we believe to 

 be nearly allied, and resist the undue influence of considerations 

 based on supposed specific distinctions. 



As a preliminary consideration, we must decide upon the 

 most convenient means of expressing the facts of geographical 

 distribution in a concise form. The recognised botanical regions 

 of the World do not serve our purpose; we are not concerned 

 with the present position of mountain-chains or wide-stretching 

 plains that constitute natural boundaries between one existing 

 flora and another, but simply with the relative geographical 

 position of localities from which records of ancient floras have 

 been obtained. For this purpose the surface of the earth may be 

 divided into six belts, from west to east, and these again into 

 22 districts. (A map is given, and 7 tables showing the distri- 

 bution of 1. Devonian and Lower Carboniferous, 2. Rhaetic, 

 3. Jurassic, 4. Wealden Floras, and of Araiicarieae, Ginkgoales, 

 Matonineae and Dlpteridinae.) 



Pre-Devonian Floras. 



The scanty records from pre-Devonian rocks afford but 

 little information as to the nature of the Vegetation that existed 

 during the period in which were deposited the Cambrian, Ordo- 

 vician, and Silurian strata. The genus Nematopliyciis, originally 

 described by Dawson as Prototaxites, and afterwards referred 

 by Carruthers to the Algae, constitutes the most satisfactory 

 example of a Silurian plant. This genus, which has fortunately 

 been preserved in such a manner as to admit of minute 

 microscopical examination, represents a widely spread algal type 

 in Silurian and Devonian seas. The tubulär Clements composing 

 the stems of some species of Nematophycus — which reached a 

 diameter of 2 or 3 feet — exhibit a regulär Variation in width, 

 giving the appearance of concentric rings of growth, as in the 

 stems of the treelike Lessonia, an existing genus of Antarctic 

 seaweeds. This structural feature presents an impressive image 

 in stone of a plant's rhythmical response to some periodically 

 recurring conditions of growth in the waters of Palaeozoic seas. 



