360 Notices of Memoirs — E. A. N. Arher — Plants as Zonal Indices. 



Sigillaria ovata Sauveur, plants •whicli liave recently been found 

 in the Cumberland Coalfield. The evidence of such restricted 

 species is not, however, the foundation of any method of zoning 

 by means of fossil plants, although it is often important as affording 

 confirmatory support to conclusions gained on entirely different 

 grounds. 



In order to establish the position of any bed in the Coal-measures, 

 it is necessary to collect and to study a number of different species 

 from it or from the associated rocks ; in other words, we must know 

 not one or two species, but a flora. The number of species need not, 

 however, be very large. Usually twenty species or even less will 

 suflSce if they belong to diversified types of plants, but the larger 

 the number the better. It is the relative abundance of certain types 

 of plants at any one horizon, and the absence of other types, rather 

 than the occurrence of particular species, which gives the solution 

 to the problem of the horizon of the bed in question. By taking 

 into account the aggregate or assemblage of plant types, the common 

 occurrence of certain classes, genera, subgenera, or species, and the 

 absence or rare occurrence of others, species which have a wide 

 range in time in Carboniferous rocks can be made to yield evidence. 

 Such species, despite their range, are found to be much more 

 abundant in some of the subdivisions of the Carboniferous than 

 in others. 



Thus the common occurrence of Lepidodendron aculeatiim, Sternb. 

 in coal-bearing strata in itself points to such beds being of Middle 

 or Lower Coal-measure age, rather than Upper, as this species has 

 been found to occur most abundantly on these horizons, and less 

 abundantly in the Upper Coal-measures. From a number of 

 separate small conclusions of this nature a general conclusion can 

 be arrived at, for which support can often be found from other 

 lines of evidence, such as the occurrence of restricted species. 

 Again, an abundance of such types of plants as Calamites and 

 Sigillaria, in association with Sphenopteris, and an absence of par- 

 ticular types of Pecopteris, Alethopteris, and Cordaites, will help to 

 distinguish a Middle from an Upper Coal-measure flora. 



Occasionally small points of disagreement with a general result 

 are found. Alethopteris Serli, Brongt., a characteristic Upper Coal- 

 measure fern-like plant, is sometimes found in the Middle Coal- 

 measures, as for instance in Cumberland. The disagreement of 

 a single character does not, however, invalidate the conclusion 

 drawn from an aggregate of characters. Such disagreements occur 

 among recent plants which are classified on very similar principles 

 to those applied here. In the recent family Scrophulariaceas, the 

 presence of five stamens in the flower is a single character con- 

 tributing towards an aggregate of characters which distinguishes 

 this family or natural order from others. But many, perhaps the 

 majority of genera belonging to this family possess only four or 

 two stamens, whereas their other characters, as a whole, clearly 

 point to close affinity with other members of the Scrophulariacege 

 possessing five stamens. It need hardly be pointed out that if all 



