M. de Blainville on the Shell of Cephalated Mollusca . 359 
&c. but that it is wanting in the Mahuya , Gecko , &e. The appear- 
ances are much more developed, comparatively, than in man ; so 
that they can be examined with the greatest ease. 
The following letter, containing additional information on the 
subject, has been received from Dr Knox : — 
“ Dear Sir, 4th August 1823. 
I think I mentioned to you, some time ago, when I had the ho- 
nour of communicating to you my discovery of the foramen cen - 
trale of the retina in reptiles, that I expected to find the same 
appearances in the Chameleon, whose eye I had not then exa- 
mined. The specimen you were so kind as send me lately, has 
verified these conjectures ; for I find, in the eye of the chame- 
leon, the foramen centrale to be remarkably large and distinct, 
exceeding greatly, proportionally, and even in absolute size, the 
same parts in man. The fold of the retina, extending from the 
foramen centrale to the entrance of the optic nerve, is equally 
apparent : there is even an appearance of a continuous fissure 
connecting these parts together ; but this appearance I ascer- 
tained to be deceptive. 
“ It is not my intention, at present, to enter into any details 
relative to this very remarkable structure in the eyes of reptiles, 
nor to offer any hypothesis relative to its cause ; yet I do not he- 
sitate to affirm it as my belief, that, ultimately, much light will 
be thrown, by this discovery, on the subject of vision in man, 
and other animals, and perhaps on the nature of light itself. 
66 I am, at the present moment, engaged in prosecuting the 
inquiry, and shall have the honour of submitting to you the re- 
sults, as soon as completed. I am, &c. R. Knox. 
“ To Professor Jameson.” 
Art. XXVIII . — Observations on the Difference of the Shell of 
Individuals of Different Sexes in the Cephalated Mollusca . 
By M. de Blainville. 
On perusing with some attention the numerous works of mo- 
dern geologists upon the distinction of fossil organic bodies, and 
especially upon that of the species of univalve shells, which have 
belonged to the class of cephalated Mollusca, the zoologist is 
