456 THE OCEAN WORLD. 
a very large shell, consisting of seven turns of the spire, studded with 
spines, and terminating in a strong beak ; the other had a shell much 
smaller. Aristotle named the last animal Auccinum. It is thought 
that the last species is to be recognised in the. Purpura lapillus 
(Fig. 285), which abounds in the Channel. Réaumur and Duhamel 
obtained, in fact, a purple colour from this species, which they applied 
to some stuffs, and found that it resisted the strongest lye. The 
genus AZurex is supposed by some to have contained the species 
indicated by Aristotle. ; . 
- Up to the present time, the production of the Tyrian purple remains 
Fig. 284.—Cassis zebra (Lamarck). Fig. 285.—Purpura lapillus. Fig. 286. — Purpura patula. 
a mystery. It was long thought this fine dye was furnished by the 
stomach and liver; but M. Lacaze-Duthiers has demonstrated 
that the organ which secretes it is found on the lower surface 
of the mantle, between the intestines and the respiratory organs, 
where it forms a sort of fascia, or small band. The colouring matter, 
as it is extracted from the animal, is yellowish ; exposed to the light, 
it becomes golden yellow, then green, taking finally a fine violet tint. 
While these transformations are in progress a peculiarly pungent 
odour is disengaged, which strongly reminds one of that of assafcetida. 
That portion of the matter which has not passed into the violet tint 
is soluble in water ; when it has taken that tint it becomes insoluble. 
The appearance of the colour seems provoked rather by the in- 
fluence of the sun’s rays than by the action of the air, The matter 
