LECTURES ON MOLLUSCA. 65 



2551,) but it has a stony operculum, with two grooves outside. The 

 pretty little African group Cottonia, have small Trochoid shells, and 

 a many-whirled shelly operculum with a central pit. Species belong- 

 ing to this type are found in the Paris Eocene beds. Fossils of Tur- 

 binoid form, which may or may not belong to this family, are found 

 in all ages from the earliest times. 



Another group, typified by Imperator, has the shell top-shaped. 

 The whirls and base are flat; the operculum thinner and oblong. The 

 shell is always roughly sculptured, and often considerably incrusted. 

 The large Pomaulax of Lower California has a channeled base, and 

 an operculum with three bent ridges. Uvanilla has a similar base, 

 with two ridges on the operculum. The New Zealand CooJcia has one 

 ridge, and a shell shaped like Modelia. The shells of Astralium have 

 a very flattened spire, with a sharp keel round the base armed with 

 spiny scales. An aberrant form of this is the Japanese Guilfordia. 

 which has a brilliant, golden nacreous texture, and a few long spines. 



Family PHASIANELLIM. (Pheasant- Snails.) 



The shells of this group differ from the Turbos in being porcellanous, 

 but not nacreous. The shelly operculum is smooth outside. The 

 shells are always smooth, and very brilliantly painted. They have 

 much the shape of Periwinkles, and the animal has a very long snout. 

 Small species are found in most warm seas, but their favorite haunt is 

 Australia. This part of the world retains the oldest fauna now living, 

 and has many points of similarity with that of the oolitic rocks. The 

 prevalence of large Pliasianellas in the European oolites and present 

 Australian seas is a striking case of similarity. 



Family TROCHID^E. (Top- Shells.) 



The animals of this family are very beautifully fringed, and the 

 shells generally highly painted. Very few excel them in the elegance 

 of the sculpture, and the beautiful shapes of their pearly mouths. The 

 shells are generally thinner than the Turbos, from which they may 

 always be known by the thin, horny, glossy operculum of many 

 whirls. The genera into which the old genus Trochus have been lately 

 divided, cannot be regarded as established until the peculiarities in 

 teeth, fringes, opercula, &c., have been examined in a much larger 

 number of species. The following are the principal groups: The 

 typical species are conical, with many whirls, the last of which often 

 bulges, with the pillar-lip twisted and concave in front. In Cardinalia, 

 the surface is sculptured, the last whirl a little narro wed-in, with the 

 pillar-lip ending in a point in front. The small conical shells^with a 

 flat pillar and square mouth, which for number and beauty might be 

 considered the principal of the groups, have been called Ziziphinus, 

 from the commonest European species; but as great confusion arises 

 from raising specific names to the generic peerage, it would be far bet- 

 ter to revive Swainson's name Calliostoma. In Pyramidea, the whirls 

 are very angular and narrow, and the pillar is sharply twisted so as 

 to approach Terebralia among the Cerites. Polydonta has the bottom 



