Dr Brewster's Reply to Mr Brooke's Observations, fyc. 361 
ward the class of acephalous mollusca. In these last, as in the 
second and third section of cephalated mollusca, the shell or pro- 
tective envelope, simple cy complex, presents only individual dif- 
ferences determined by age or by some accidental circumstance ; 
but, in the first, the case is different ; and I am already assured, 
that, in several genera, whose animals I have seen, the shell of 
female individuals is always more bulging, especially in the last 
revolution, which renders the orifice more widened to the right, 
and that the spire is less attenuated and less pointed than in the 
male individuals. This is particularly well marked in the Buc- 
cina, Cyclostomata, Paludinre, andin the Ampullariae, which, it is 
true, differ so little from this last genus, that I am assured from 
dissection, that it would be difficult to separate them generally 
from it. Hence, it would appear, that, in future, conchologists 
would do well to attend, when distinguishing species of shells, to 
the differences which I have here pointed out *. 
Art. XXIX. — Reply to Mr Brooke's Observations on the con- 
nexion between the Optical Structure of Mmerals and their 
Primitive Forms. By David Brewster, LL. D. F. II. S. 
Lond. & Sec. R. S. Edin. 
In the year 1817, I had occasion to announce to the Royal 
Society of London the existence of a physical law, by which the 
Primitive Forms of crystallized bodies could be hf erred from 
the number of their axes of double refraction f . This law was 
established by a comparison of my own observations on the po- 
larising structure of crystals with Haiiy's Table of Primitive 
Forms ; but as one of H ally's divisions contained a greater num- 
ber of erroneous than of correct determinations, I was of course 
obliged to state as exceptions to the law what were really ex- 
amples of it, and to give as examples of it what actually consti- 
tuted exceptions. 
In a Memoir, printed in the Wernerian Transactions , vol iii, 
p. 50., I pursued this subject to a greater length ; and in a sub- 
sequent Memoir, vol. iii. p. 337., laid before the same scientific 
body, I exhibited the law in its most general form, and freely 
~ Journal de Physique , 18?!?, 
f Phil Tram ■ 1818. 
