N^,^ LECTURES ON MOLLUSCA. 177 



it looks shapeless, like a chip of rosewood; within, however, it is seen 

 that it has been formed on the usual concentric plan, but with the 

 nucleus elongated, and turned towards the outer lip of the shell. The 

 name of the principal genus is derived from a crimson dye which many 

 of the species exude when pressed. It was not, however, from these, 

 but from the Murex brandaris and M. trunculus of the Mediterranean, 

 that the ancients obtained their celebrated Tyrian purple. Cavities in 

 the rocks, with heaps of the broken shells, where the mollusks were 

 sacrificed to dye the robes of the nobles, are still seen on the shores of 

 the Morea and Levant. 



The shells of this group reproduce many of the forms of the Muri- 

 cids, but with the chip, instead of the claw-shaped operculum. Thus 

 Cerastoma has regular varices like Murex and Vitidaria ; irregular 

 ones like Trophon. Rhizocheilus has generally been confounded with 

 Muricidea. Chorus presents the shape of Chrysodomus, and liapana 

 of Pyrula. Iopas takes the place of Pisania; the wry-mouthed Rici- 

 nula of Engina; and Nitidella represents Anachis and the Columbellas. 

 The true Purpura has a peculiar scooping out of the pillar-lip. This, 

 when exaggerated, and at the same time the body whirl greatly 

 enlarged at the expense of the spire, produces the common Concholepas 

 of the Peruvian coast, which at first sight might be taken for a limpet. 

 In Monoceros, a genus almost peculiar to the Avest coast of America, 

 and ranging from California to Cape Horn, a sharp spine is developed 

 at the base of the outer lip. The same is seen in Chorus, Cerastoma, 

 and Concholepas ; and may be looked upon as a west American pecu- 

 liarity. 



In the Rapana group, Melapium represents the Pyrula melongena, 

 and the delicate Papa shells the Ficulas. The Pseudoliva is clothed 

 with a coarse epidermis, and has a channel running spirally outside 

 the base of the shell, the use of which is not known. In the angular 

 Cuma tectum and in Purpura columellaris , there is a hump which runs 

 along the middle of the pillar lip. 



The purple-shells frequent rocky shores all round the globe, and are 

 generally very prolific. They feast on bivalves, periwinkles and other 

 shell-fish. Some of them are very sedentary in their habits, especially 

 the Rhizocheils, which clasp round the stems of corals and prey upon 

 the Polypes. These often have the breathing canal almost rudimentary. 



The Magilus, which used to be considered an Annelid, and afterwards 

 a Vermetid, is perhaps a degraded member of this group. When young 

 it has a white, globular shell, shaped like Natica. It establishes itself 

 among the Red Sea Polypes ; and as the corals grow upwards, so does 

 the Magilus, forming a solid, irregular tube, with a keel to represent 

 the canal. Leptoconchus resembles its young state, but with a slight 

 notch, and no operculum. The Magilus, having plenty of lime to eat, 

 fills up its spire and the forsaken part of its tube with solid shelly 

 matter. 



Family Nassid^e. (Dog-ivhelks .) 



The Nassas have small, compact, highly sculptured shells, with a 

 sharply twisted notch, through which the long curly siphon protrudes. 

 There is generally a strong lump on the inner lip. The animal has 

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