Miscellaneous. 305 
covers the foot. ... But how does a flat oval or circular surface 
produce a material which rolls into a spiral, often in a very regular 
manner, and sometimes forming a great number of turns? It is 
a question which it really seems to me very difficult to answer, 
perhaps especially because it has not been sufficiently studied.” 
In 1829 Dugés endeavoured to fill up this gap, but, considering 
only opercula detached from the animals, he arrived at the false 
conclusion that the operculum is a production of the mantle. The 
authors who have followed him have scarcely attempted to do more 
than try to find ont with what part of the Acephala the operculum 
was homologous ; but, as they had no basis for their argumentation, 
they arrived at the most contradictory results, some regarding it as 
the homologue of the second valve of the Lamellibranchs, others, 
like Lowén, comparing it with the byssus. 
The foot alone is implicated in the formation of the operculum, 
and I have set myself to determine precisely what parts of this 
organ excrete the material, and how the growth of the operculum 
takes place. Contrary to what is generally supposed, I have 
ascertained that the whole surface of the foot does not take part in 
the production, but only a very clearly defined small portion. 
I shall give the name of the columellar margin of the operculum 
to that which lies on the side of the columella when the animal is 
withdrawn into its shell, and that of parietal margin to the opposite 
part. The operculum presents an internal and an external surface; 
it is necessary to distinguish them, as they have not the same mode 
of formation. 
Let us first of all see to the external part. It presents on its 
surface strie of variable form according to the genera under 
examination. On examining carefully a Littorina, a Murex, or a 
Purpura, we observe, quite close to the columellar margin of the 
operculum, a small transverse fissure which penetrates about 
1 millim. into the thickness of the columellar muscle, and which 
occupies the whole length of the foot. The walls of this pedal fis- 
sure are lined with a peculiar epithelium, folded, or rather goffered, 
so as somewhat to resemble the polypary of a Meandrina. We 
see within it a very delicate and very flexible transparent lamella. 
With a fine needle this lamella may be taken out, when after 
remaining for some time in the air, it dries and acquires a horny 
appearance. 
By making sections we find that the epithelial cells of the fissure 
excrete a structureless material, of a yellowish colour, and strongly 
refractive, which, by ag elomeration, constitutes the hyaline lamella. 
The latter issues from. the pedal fissure and adds itself to the old 
operculum. 
The newly formed parts apply themselves to the epithelium 
situated between the fissure and the parts of the operculum which 
are already fixed. At this poiut the epithelial cells are but feebly 
adherent to each other, and only by their basal part, a remarkable 
exception among epithelia, From this it results that the still 
plastic opercular material invests these cells, and even diffuses itself 
