6S 



ME. D. DATIES OX THE ECOLOGY OF T^'ol. Ixxvii, 



Filicales and Ptericlo sperms dominate other classes of plants 

 three times, in the Upper Yard, Six-Foot, and Xo. 3 Ehondda 

 Seams. 



Cordaitales dommate the other classes once only, in the Five- 

 Foot Seam, the lowest horizon. 



It is found that, when a genus becomes common in the field, 

 the number of the species generally lessens. For instance, if we 

 take Cala mites, A^teropliyUites, and Annularia from the Equi- 

 setales, the two genera Lepidodendron and SiqiUcrria from the 

 Lycopodiales, and the genera S^eurojpteris. Splienopteris, Feco- 

 fieriB, Jlariojpteris, and Aletliojpteris of the fern and fernlike 

 plants, the general graphs show an mcreased number of these 

 different genera of plants and a decrease in the number of species 

 belon^ins^ to the same 2:enus at the same horizon. 



c? o o 



Taking the ten horizons on which the ecology and the graph 

 of convergence and divergence of species has been based, the two 

 graphs show that there is an increase in the number of species and 

 a decrease in the number of the corresponding genus in the field. 

 This has happened at the ten horizons on 34 occasions. The 

 converse of this statement : that is, a decrease in the number of 

 species and an increase in the genera, has occm-red on 11 occasions. 

 An increase of species following an increase of genera happens on 

 13 occasions, and a decrease of species followed by a decrease of 

 genera occurs on 11 occasions. 



The question that natm-ally arises from the preceding statements 

 is, whether plants would have a tendency to remain quiescent in 

 development of differentiation during a period that was favourable 

 for their existence : and also, would unfavourable physical conditions 

 cause the plants to make an extra effort towards adaptation and 

 bring about the development of new species 'r 



Y. StEATIGEAPHICAL CoXSIDEEATIOyS : THE ExiSTEXCE OE A 



B];eax between the Westphalia^ axd the Staeeoedia>"" 

 Seeies. 



"SYith the lithological change in the sti-ata at the Xo. 3 Ehondda 

 Seam, where sandstones begin to take the place of shales, it is 

 observed that pebbles of quartz, sandstone, and coal occur as thin 

 conglomerates of a lenticular character. Even the sandstones 

 contain smooth and well-rounded pebbles of coal, varying in size 

 from a thrush's eg^ to a big orange. 



Foul' samples of these coal-pebbles from the sandstone above 

 Xo. 2 Ehondda Seam, and three samples of Xo. 2 Ehondda coal 

 which lies immediately below the sandstone fi-oni which the coal- 

 pebbles were taken, have been subjected to chemical analvsis bv 

 Mr. E. M. Bowen. 



The samples of coal-pebbles, like the samples of Xo. 2 Ehondda 

 coal itself, were fomid to be coking coal. The analyses varied in 

 the coal-pebbles, and are as follows : — 



