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.
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.
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y Monad ; D, D vad atom.
