636 MOLLUSCA— CLASS IV.— GASTROPODA. 



of the animal is plug-like and quite useless for crawling. In the genua 

 Tenagodes the tubular shell is fissured for almost its whole length. 



The CERiTHiiDiE have long shells like the Turritellidte, but the mouth ia 

 not so round and ia interrupted by the anterior and posterior canals, the 

 former of which is usually bent to the left. The operculum is horny. The 

 whorls are generally sculptured, or ornamented with ribs, tubercles, or 

 spines. They occur in all parts of the world and dwell in the shallow waters 

 of coasts and estuaries, some even advancing up the rivers into fresh waters. 

 There are three British species. 



The wing-shells, Strombid^, Aporrhaid^,, and Strdthiglariidjs, are 

 noted for their strong shells, with very short spire and widely expanded outer 

 lip, which is often drawn out into curved spines. The fountain-shell, 

 Stromhus gigas, of the West Indies, the scorpion-shells, Pterocera, and the 

 pelican's-foot shell, Aporrhais, are familiar examples. The operculum, 

 which is rudimentary in all, is horny and claw-shaped : in the Strombidee its 

 outer edge ia serrate. 



In the CvPR^EiD.E, or cowries, the outer lip of the adult shell is curled 

 inwards, blocking still further the already narrow aperture, which is, 

 moreover, edged with numerous fine teeth, whilst the short spire is almost, 

 if not quite, concealed. The young shell, on the other hand, has a distinct 

 spire and thin, sharp lip. The mantle when the animal is crawling is greatly 

 extended, enveloping the shell, which thus acquires that beautiful polished 

 surface for which the cowries are noted. 



The DoLiiniE are, for the most part, large, thin, and rather globose shells, 

 the fragile whorls of which are strengthened by their walls being corrugated. 

 In Pirxda the shell is stouter, and furnished with a long anterior canal. 

 There is no operculum. They are denizens of tropical seas. 



The CASsiDiDiE, or helmet-shells, resemble the preceding, to which they 

 are closely allied, and with which they are found. Their shells, however, 

 have the margin of the outer lip inflected, or thickened and crenulated, and 

 they furthermore possess a fan-shaped, horny operculum. The under-layers 

 of the shell in Stromhus, Gyprcea, and Gassis being of a difi'erent colour to the 

 upper onea, these shells lend themselves especially to the art of the cameo 

 engraver : the best results are generally those obtained from the Cassis rufa 

 of West Africa. 



The LoTORiiD^ are best typified by the great triton-shell (Fig. 12), which 

 the sea-gods in mythical pictures are represented blowing. It is a fine large 

 shell, with slender pointed spire and numerous varices, resulting from the 

 formation of periodic mouths. The aperture is large and oval, whilst both 

 inner and outer lip are denticulate. The periostracum in this family ia 

 usually stout and fibrous : the operculum is horny and claw-shaped. 



The most highly specialised of all the T;enioglossa are certain pelagic 



forms formerly placed in a separ- 

 ate group, and called Heteropoda 

 or Nucleobranchiata. The 



animals are remarkable for their 

 transparency, all the various 

 organs being perfectly visible 

 even through the shell. They 

 swim upside down in the water, 

 t^g. i6.-CA,uNiR.A. ^^e flattened foot acting as a fin. 



1 wo families are distinguished : — 



