634 MOLLUSC A CLASS IV. GASTROPODA, 



up with a shelly deposit termed the callus. The operculum is either horny 

 or shelly. The animal of Natica is blind and has an extension of the fore- 

 part of the foot that projects over its head when the animal is crawling ; 

 extensions of the side of the foot also partially envelop the shell. The 

 Naticas prey on bivalves, boring a small hole right through the shell. 

 Several species are British. 



The LAMELLARIID.E have a delicate shell over which the mantle is reflected 

 to a greater or less degree. The aperture is very large and rounded, and 

 there is no operculum. 



The TRICHOTROPID.E have a conical shell ; the whorls are keeled, and 

 the periostracum, where it covers these, is fringed with hairs. They are 

 northern forms and come from moderately deep water. The operculum is 

 short and claw-shaped. 



The NARICIDJE have shells shaped like Nalica, but covered with a velvety 

 periostracum, whilst the shell itself is marked with spiral and transverse ridges, 

 forming a kind of network sculpturing. The operculum is thin and horny. 

 The majority of the XENOPHORID^ may be recognised at once by their 

 curious habit of building into their own shells 

 those or parts of others, stones, corals, or what- 

 ever may happen to be lying on the sea-bottom 

 where they dwell (Fig. 15). 



The shell in the CAPULID^E is limpet-like. 

 In CapuLus, the cap-of -liberty shells, it has a 

 little curly apex ; whilst in the cup-and-saucer 

 limpets (Crucibulum, etc.) there is an internal 

 shelly projection which serves for muscular 

 attachments. 



The HIPPONICID.E have also limpet -like 

 Fig. 15. XENOPHORA. shells, but the mouth is closed by an accu- 



rately-fitting shelly plate attached to some fixed 



object. This plate is secreted by a kind of second mantle that envelopes 

 the under surface of the animal and represents the foot, of which there is 

 none properly so-called. A great horse-shoe-shaped muscle holds the shell 

 to its base. 



The four next families are all dwellers on the land, and in place of the 

 gill, which is not developed, the pallial cavity is converted into a lung, as 

 in the common snail. The animals have a distinct snout. 



The TRUNCATELLIDJE and ARICULID^ are diminutive forms with cylin- 

 drical, many-whorled shells having a blunt apex, and closed with a spiral 

 operculum. 



The CYCLOSTOMATID.E have spiral shells, sometimes top-shaped, sometimes 

 globose, closed by a spiral operculum. One species of Cyclostoma is found 

 on our chalk downs. 



The CYCLOPHORID^E vary much in the shape of their shells, some having 

 high spires, whilst others have none at all. The mouth is always circular, 

 and closed with a tightly-fitting operculum, in which the nucleus is generally 

 central. To enable the creature to breathe, when fast shut in its shell, there 

 is often either a notch in the peristome or an aperture a short distance from 

 the margin, that is occasionally prolonged into a tube. A few examples 

 occur in Southern Europe, but the majority are found in the tropical regions 

 of the Old World. 



The succeeding five families are all fresh or brackish-water forms. 



