632 MOLLUSCA-CLASS IV.— GASTROPODA. 



The PLBUEOTOMiDiE, Or slit-shells, have conical, spiral shells with a notch 

 ill the outer lip at the periphery. The typical genus I'leurotoma is one 

 of the rarest of living sea-shells (only a few 

 specimens have ever been dredged), but they 

 include one very fine example measuring over 6 in. 

 in height (Fig. 14). There are two gills and the 

 mantle is slit in front. The shell is pearly within, 

 and there is an operculum. 



The HaliotiDjE, or ormers, have an ear-shaped, 

 flat, spiral, pearly shell, with a row of perforations, 

 a very wide aperture, and no operculum. The 

 animal has two gills, of which the right one (left 

 in position) is the smaller. 



In the families next succeeding the mantle is 

 ^'''' "AlA^NsomAN?""'" entire, there is no slit or perforation in the shell, 

 and only one gill remains, the (originally) left one. 

 The Tkochid.s: have spiral shells of conical form, and very largely com- 

 posed of nacre ; they are frequently used for necklaces after the non-pearly 

 outer coat has been removed. Some of the smaller examples are common 

 on our coasts, but the finer and more handsomely coloured ones all come 

 from tropical seas. The operculum is circular, horny, and spiral. 



The SiOMATiDiE are for the most part pearly shells with a minute spire 

 and large aperture ; the operculum is like that of the Trochidce. 



The DelphindlidyE are flat-topped, spiral shells, having few whorls ; the 

 umbilicus is wide and the angles of the whorls rough or spiny ; the interior is 

 pearly. They occur in tropical seas. 



The Cyclosteematid^ are small, sometimes minute, nearly discoidal sheila, 

 with cross-ribbing on the whorls. They are not pearly within. 



The TuRBiNiDiE include large and very pearly shells, much used as orna- 

 ments. They have a moderate number of rounded whorls, sometimes very 

 richly coloured. The operculum is calcareous, and sometimes very thick 

 and heavy, with the outer surface frequently beautiful!}' sculptured or 

 coloured : one kind, called cat's-eyes, has been used for necklaces. The 

 Turbinidffi are tropical forms. 



The Neritopsidje have solid shells much resembling Nalica in shape. 

 There is only one living species known. 



The NBKiTiDiE have solid and rather globular shells of not many whorls 

 and very short spire, or none at all ; the columellar lip is crenulate or 

 toothed, and the shelly operculum is articulated. The genus Ncritina 

 occurs both in the sea and in fresh water. 



The TiTiscANiiDiE are shell-less : one species only is known, which cornea 

 from the Pacific. 



The PiiGSPERi'iNiDiE, HELiciNiDiE, and HYnROCENiD,!: are terrestrial 

 members of the sub-order, in which the gill is absent and the gill cavity 

 transformed into a pulmonary chamber, as in the snail. The shell is more 

 or less globular, and mostly of a yellow tint. All three are tropical families, 

 and occur, the first two in the New World, the laat in the Old. 



Order 11. — Pectinieranchiata 

 (Sometimes termed Monotocardia, the heart having only one auricle). 



In this order, which includes the majority of the marine Gastropods, 

 only one gill remains, the (originally) left one. The greater number, 



