358 SECTIONAL TRANSACTIONS. — C, D. 



are largely based on the arrangement of the atoms in crystal- 

 line solids, since this arrangement affords an insight into the 

 nature of the forces acting between the atoms. In such 

 compounds as sodium chloride, it is probable that the atoms 

 exist as ions of sodium and chlorine, and that the crystal is 

 held together by the electrostatic attractions of these ions, 

 thus accounting for the fact that there is no grouping of the 

 atoms into molecules in the solid. In other compounds, such 

 as those of two electronegative elements, the molecular 

 arrangement persists in the solid state and the chemical com- 

 bination appears to be of a different type from that of sodium 

 chloride. A consideration of the distances between the 

 atomic centres in crystals supports the conception of the 

 two types of chemical combination. 



13. Mr. D. C. Evans. — The Ordoviceo-Valentian Succession 



in North-east Pembrokeshire and North Carmarthen- 

 shire. 



14. Mr. David Da vies. — Palaontology of the Westphalian 



and lower part of the Staff ordian Series of the Coal 

 Measures as found at Clydach Vale and Gilfach Goch, 

 East Glamorgan. 



Eecorded : 45,000 plants ; 1000 shells ; 2 insects and 

 one fish scale. Plants yielded 154 .«pecies, shells 6 species, 

 insects 2 species, fish scale one ; 45 of these are new 

 to South Wales and 7 new to Britain. Ecology of ten 

 horizons : Equisetales predominate in four ; Filicales and 

 Pteridosperms in three ; Lycopods in two, and Cordaitales in 

 one horizon. When Lycopods predominate. Fern and Fern- 

 like plants are weak, and vice, versa. 37 per cent, plants are 

 common to both series ; 31 per cent, distinctly Staffordian ; 

 32 per cent, distinctly Westphalian. The Pennant Sandstone 

 produced smooth round coal pebbles, giving evidence of a 

 geological break. A significant feature is the appearance of 

 new species at this period. 



SECTION D.— ZOOLOGY. 



(For references to the publication elsewhere of communications entered in 

 the following list of transactions, see p. 381.) 



Tuesday, August 24. 



1, Presidential Address by Prof. J. Stanley Gardiner, 

 P.R.S. (see p. 87). Followed by a Discussion. 



Afternoon. 



2. Prof. J. Stephenson. — The Polyphyletic Origin of Genera 



in the Oligoch(sta, and its bearings. 



3. Prof. P. Fauvel. — Tlie Affinities of the Annelidan Fauna 



of the Abrolhos Islands. 



