320 



THE BLACK OLIVE. 



is one-eighth the natural size. About fourteen species of Tun-shells are known, all inhabiting 

 the wanner seas. 



HELMET SHELL. -Cassis glauca (Sma 



The beautiful Helmet-shells are tolerably thick and solid, and their external surface is 

 covered with bold ridges, marking the periodical growth. These ridges are technically called 



"varices." All the Helmet-shells 

 are natives of the tropical seas, 

 and appear to prefer the shallow 

 waters near the coast. Several of 

 these shells are employed by the 

 engravers in the manufacture of 

 cameos, the differently colored lay- 

 ers producing most exquisite ef- 

 fects when cut by a judicious oper- 

 ator. The colors vary greatly in 

 the different species, and some- 

 times there is a slight variation 

 even in different individuals be- 

 longing to the same species. Cam- 

 eos, for example, that are cut from 

 the Horned Helmet-shell (Cassis 

 eornutd) are white, upon a ground of rich orange; those that are made from the Warty 

 Hklmet-shell (Cassis tuberbsa) are white, on deep dark red; the cameos formed from the 

 shell of the Ruddy Helmet (Cassis rufd) are saffron-yellow on warm orange. Another 

 beautiful species, called the Queen Conch (Cassis madagascariensis), furnishes a white 

 cameo on a claret-colored ground. 



The next illustration is a dark smooth shell, represented as crawling on the ground, and 

 partially enveloped in the spotted textures of the living creature. 



This is the Black Olive, so called on account of the jetty blackness of its exterior, and 

 the oval, rounded form, which is not unlike that of the fruit whose name it bears. The genus 

 Oliva is a very large one, comprising more 

 than one hundred species, and found in all 

 the warm and tropical seas. As may be 

 seen by the figure, the mantle is furnished 

 with two large lobes, that nearly meet over 

 the back while the animal is moving, and 

 which throw out certain filamentary pro- 

 jections, that look very like tenacles in the 

 wrong place. The foot is very large — so 

 large, indeed, that the shelly is partly 

 buried in its soft material — and the eyes 

 are, as may lie seen in the figure, placed 

 before the middle of the tenacles. 



Owing, probably, to the great devel- 

 opment of the foot and mantle, the Olives are active creatures, gliding about with tolerable 

 speed, burying themselves in the sand when the tide leaves the shores on which they are 

 creeping; and if laid upon their backs, they can easily resume their original position by the 

 use of the spreading foot. In spite of their elegant and harmless aspect, the Olives are 

 predaceous and hungry creatures, and can readily be captured by the simple process of 

 tying a piece of meat to a line, lowering' it towards the spot where the Olives are creep- 

 ing, and hauling it up at intervals, carrying with it the various niollusks that have attached 

 themselves to the bait, and do not think of loosening their hold until too late. 



The shell of the Black Olive is beautifully polished and of a deep rich black, through 



BLACK OLlVii.— Oliva mauritana. (Natural size.) 



