252 
Proceedings of the Royal Society 
The dentine extended down the fang to the sides of the narrow cleft 
at its base, which communicated with the pulp cavity. This cavity, 
bounded by the dentine, was contracted at the base of the fang, 
but dilated into a considerable space in the body of the tooth. The 
fang was invested by cement, but between the cement and dentine 
a layer of modified vaso-dentine was situated which increased in 
thickness in the lower part of the fang, whilst the dentine became 
thin. The structure of this tooth was then compared with that of 
the adult Mesoplodon Sowerbyii described by Professor Ray Lan- 
kester.* 
The teeth both of M. Layardii and Sowerbyii in their non-erupted 
stage do not materially differ in structure from the ordinary human 
or carnivorous teeth, for the crown is covered by enamel, and the 
fang by cement, whilst the great body of the tooth consists of den- 
tine, in which is a well-marked pulp cavity. The exceptional struc- 
ture of these teeth in the erupted stage is due to the disappearance 
of the enamel from the crown, to the cessation in development of 
the ordinary dentine, and to the excessive formation in the adult 
Sowerbyii of osteo dentine, and in Layardii of modified vaso-dentine, 
which cause the fang to assume unusual dimensions. 
The following Gentleman, having been duly recommended 
and balloted for, was elected a Fellow of the Society: — 
James Abkrnethy, V.P. Inst. C.E., Prince of Wales Terrace, 
Kensington Garden, London. 
Monday , Ibtli June 1879. 
Professor MACLAGAN, Vice-President, in the Chair. 
The following Communications were read : — 
1. Atomicity or Valence of Elementary Atoms : Is it constant 
or variable ? By Professor Crum Brown. 
‘Quarterly Journal of Microscopic Science,” 1867, vol. vii. 
