294 Prof. W. J. Sollas. 



on some such view as that put forward by Pelseneer, and this is further 

 confirmed by evidence, during the stages in which torsion is being 

 effected, of an actual twist of the oasophagus, which keeps pace exactly 

 with the torsion of the whole body. 



Various monstrosities were found, which all lent support to 

 Pelseneer's hypothesis rather than to Biitschli's. Chief of these was a 

 well-developed untwisted form, with symmetrical mantle cavity and 

 kidneys, and a strong tendency towards exogastric coiling of the 

 visceral hump. 



With regard to the ontogenetic cause of torsion merely negative 

 evidence was found. That it is almost certainly not due to antagonism 

 of growth between the foot and the visceral hump is shown by a com- 

 parison between normal forms and monstrosities. 



" On the Intimate Structure of Crystals. Part V. Cubic Crystals 

 with Octahedral Cleavage." By W. J. SOLLAS, D.Sc., LL.D., 

 F.K.S., Professor of Geology in the University of Oxford. 

 Eeceived April 10, Read May 23, 1901. 



During the three years that have elapsed since the last part of this 

 contribution was communicated to the Society, continued reflection 

 has served only to confirm my belief that it is to the molecular 

 volumes of crystalline matter we must turn for insight into its 

 structure. There are several points of detail in which the results 

 already obtained might be usefully modified, but the discussion of 

 these may safely be postponed for the present, while we pass on to 

 more important matters. For we have now reached a critical point 

 in our enquiry, we propose to investigate more complex compounds 

 than those hitherto considered, compounds also distinguished by a 

 different crystalline structure. If we find, and I think we shall, that 

 the constituent atoms of these compounds retain the specific volumes, 

 which were determined from a study of very different cases, then our 

 hypothesis will begin to appear less speculative, and we shall be able 

 with greater confidence to extend our enquiries in other directions. 



Triatomic compounds of the type HoO are frequently represented 

 graphically in the way shown by fig. 1. Molecules in which the atoms 



FIG. 1. FIG. 2. 



y Monad ; D, D vad atom. 



