260 . Royal Society :— 
nishes, f the focal ocu of the virtual image formed by the facet 
lenses of the objec 
ds y) 
ox (- LE 
e appendix refers to plates illustrating the mechanical piit i 
ments for the discrimination of eidola and true images, and f 
The plates also show the course of the o opt tical pencils, spurious 
disks of residuary aberration and imperfect definition, as w 
some examples of “high-power essai? of t odura and 
Lepisma beading, as well as the amount of aroliGonticis obtained by 
camera-lucida outline drawings of a given scale. 
June 16, 1870.—General Sir Edward Sabine, K.C.B., President, 
in the Chair. 
** Observations on the Mode of Growth of Discoid and Turbinated 
Shells.” By ALEXANDER MacaLisTER, Professor of Zoology, 
University of Dublin 
A most interesting paper on the iiis forms of turbinated and 
UM shells was published by t ev. Canon Moseley in the Phi- 
losophieal Transactions for 1838, m 331, i in which some important 
arse were noticed regarding the geometrical construction of shell- 
forms. The author of that paper describes discoid shells as generated 
by the revolution around a central point of the perimeter of a geo- 
metrical figure, which latter, although regularly increasing in size 
yet remains always geometrically similar in form he producing 
figure in many Gasteropodous Mollusks is represented by the o oper- 
culum ; and in all it may be recognized by making a vertical section 
in the ‘plane of the radius vector. A turbinated shell is similarly 
v. Mr. Moseley gives, as illustrations of these points, mea- 
Aera of Nautilus pompilius, Turbo phasianus, Turbo duplicatus, 
and Buccinum subulatum, and describes many interesting. particulars 
: nene the formation and growth of the operculum in different 
shel 
This subject does not seem to have attracted much attention from 
utr as, with the exception of a notice in Professor Good- 
sir's lecture ** On the Use of Mathematical Modes of promit ord 
Organic Ponce” £ it is not, to my knowledge, referred to by any 
writer on zool 
cal importance might be drawn from these valuable geometrical 
observations, and more especially to determine whether it might be 
* Goodsir's ‘ Anatomical Memoirs,’ vol. ii. p. 209. 
