In size and shape this very common creature is rather like the dog periwinkle. But its shell is white in colour instead of bluish black, and generally has two or three bands of light yellowish brown running round it. You may often find it crawling about on the weed-covered rocks when the tide is out. [Illustration: PLATE IX 1. THE DOG WHELK. 2. THE STING WINKLE. 3. THE PERIWINKLE. 4. THE DOG PERIWINKLE. 5. THE PURPURA.] The purpura is quite a famous creature, because of the use which was made of it by the ancient Romans. I dare say you know that in days of old the colour of purple was very highly valued; and among the Romans only members of the royal family were allowed to dress in purple garments. Now this purple dye was obtained from the purpura. Inside its body this creature has a little bag which contains about a drop of a thick white liquid, rather like milk. Certainly it does not look in the least like purple dye. But if you were to squeeze it out on to a sheet of white paper, and to place it in the sunshine, you would very soon see that it was changing colour. In a few minutes’ time it would have turned to yellow. After a little time longer you would notice a blue tinge creeping into the yellow, and turning it to green; and by degrees the blue would become stronger and stronger, till the green disappeared. At last a crimson tinge would creep into the blue and turn it to purple; and this would be exactly the same as the famous purple dye which the ancient Romans valued so highly. The eggs which are laid by the purpura are very curious indeed, for they are fastened down to stones by little stalks; so that each one looks rather like an egg-cup with an egg inside it. And inside each of these eggs are several little purpuras instead of only one. PLATE X THE SEA SNAIL (1) This is one of the very commonest of all the shell-bearing molluscs. You may find it crawling about in numbers all over the weed-covered rocks which are left bare as the tide goes down. Its shell varies very much in colour, for it is sometimes bright yellow, and sometimes pale yellow, and sometimes olive green, and sometimes brown, and sometimes almost black. Indeed, you might almost think that there were half-a-dozen different kinds of these sea snails instead of only one. These creatures have tooth-ribbons set with hundreds of tiny hooked teeth, just like those of the dog whelks, and they use them in feeding upon the leaves of sea-weeds in just the same way. PLATE X THE WENTLETRAP (2) The Wentletrap is one of the most beautiful of all the shells which are to be found upon the shore. Indeed, I really think that it is quite the most beautiful. For the high ridges which stand out so boldly run round and round it in the most graceful curves, and the whole shell looks just as if it had been carved out of ivory. [Illustration: PLATE X 1. THE SEA SNAIL. 2. THE WENTLETRAP.]