64 LECTURES ON MOLLUSCA. 



resembling a fresh-water Crepidula with an operculum. These are 

 the Navicellas, the operculum heing small, and imbedded in the foot. 

 Pelex is a little New Zealand shell, brought home by the United 

 States Exploring Expedition, in which the apex is on one side. 



All the Nerites have the power of absorbing the inner whirls of the 

 shell, which makes the transition from the spiral to the straight forms 

 less extraordinary. The teeth are arranged in very complicated pat- 

 terns, the inner rows being of many different shapes, flanked by 

 numerous rows of hooks at the sides. 



The great bulk of the Scutibranchs consist of the Top-shells, form- 

 ing the staple of Linnaeus' two genera Trochus and Turbo. The ani- 

 mals are all formed on the same type; and are known by the beautiful 

 fringe and feelers round the foot and head, the long tentacles and eyes 

 behind on stumps, and the long and very complicated tongue-ribbon. 

 Although the animals can be easily obtained and examined, being 

 very generally found between tide-marks, the beauty of the shells has 

 generally engrossed the attention of collectors; and we are left in 

 ignorance how far the observed differences in these are coordinate 

 with distinctions in the living creatures. The divisions, both into 

 families and genera, are therefore for the most part artificial ; but are 

 rendered necessary in consequence of the great multitude of species. 

 They are found in all seas, from the tropics to the frozen ocean. 

 When their beautifully sculptured and delicately painted shells are 

 found in company with the dull Periwinkles, and their highly orna- 

 mented bodies are compared with the plain forms of the latter, it is 

 difficult to realize the fact of their greatly inferior organization. 



Family TURBINID^:. 



The shells of this group are all tropical, or nearly so. They reach 

 the Mediterranean, but not the British or temperate American seas. 

 They are distinguished by a very thick shelly operculum of few 

 whirls. The under layer of the shell is brilliantly pearly. 



The Turbo group have rounded whirls and a circular mouth. The 

 large species are imported in great quantities to be polished for orna- 

 ments ; the hemispherical opercula used formerly to be regarded as a 

 charm for sore eyes. The typical species have a smooth, or slightly 

 granular operculum. In T. sarmaticus, the surface is made up of 

 large granules. The Snake-shell group, which abound in the Pacific 

 islands, have a very rough outside, and a chink at the pillar. The 

 shells of Marmorostoma are flattened, with a deep umbilicus, and a 

 groove round the operculum which has more whirls than usual. 

 Ninella is broad and thin, with a wide, channeled umbilicus ; the 

 operculum is nearly flat, with ridges like the human ear. The shells 

 of Callopoma are like the typical forms ; but the opercula are deeply 

 grooved, with beautiful granular ridges. They are peculiar to west 

 tropical America. To the south of Callopoma, on the west of South 

 America, is found Prisogaster, with the shape and dull aspect of Lito- 

 rina, but a shelly, sharp-edged operculum of few whirls. The New 

 Zealand form Modelia has the general shape of Zizipkinue, (a species 

 of which is unfortunately figured in this place in Chenu's Manual, f. 



