GASTROPODS. 373 



one central or rhachidian, and one lateral on each side ; the latter, however, are 

 wanting in certain groups. The respiratory siphon is rather long, fitting into the 

 anterior canal or notch of the aperture. The eggs of these molluscs are deposited in 

 strong leathery capsules, each capsule often containing a large number of ova. The 

 genus Yetus, however, which is viviparous, produces a comparatively small number 

 of young. All have shells, which in nearly all cases are capable of sheltering the 

 entire animal. The principal families in this group are the Olividce, Harpidce, 

 Marginellidce, Volutidce, Mitriclce, Fasciolariidce, Turbinellidce, Buccinidce, 

 Nassidce, Columbellidce, Muricidce, and Corolliophilidce. 



The Olives (Olividce) are mostly cylindrical shells, often beautifully orna- 

 mented with markings of various patterns, and always having a highly glossy or 

 enamelled appearance. The aperture is narrow or notched in front, and the 

 columella is not strongly plaited as in some of the families which follow 

 (Marginellidce), but there 

 are numerous, slight, 

 oblique folds, some of 

 which pass round the base ;:% 

 or anterior part of the 

 shell, which is thus marked 

 off from the rest of the 

 surface into a distinct area. Ip 

 The animal has a very ^gg 



large foot, capable of :;/, 



covering the shell to a BLACK 0LIVE) olim maum (nat size) _ 



great extent, pointed 



behind, and with its anterior portion (propodium) divided into two lobes. The 

 head is small, with pointed, slender tentacles, and the eyes about midway along 

 their outer sides. The mantle is produced in front into a slender appendage, 

 which protrudes with the breathing -siphon through the notch of the aperture ; 

 posteriorly it terminates in a thread-like process, which passes up the channelled 

 suture of the spire. There is no operculum. The olives are very active, and burrow 

 in the sand in search of bivalves ; and are sometimes seen in countless numbers, 

 the sands at low tide for miles being covered with them. Although about one 

 hundred and fifty to two hundred species, mostly tropical, have been described, 

 a few extend to more temperate seas as far north as Japan, and southward to 

 Patagonia. The olives are frequently used in the manufacture of shell-ornaments, 

 The genus Olivella differs from the typical Olivet in having neither tentacles nor 

 eyes, and the spire of the shell is longer and more pointed. Ancilla, another 

 important genus, comprises a number of polished species, which are nearly always 

 of a uniform white, yellow, fawn, brown, or reddish colour, and without the 

 markings of the olives. 



The family Harpidce contains but a single genus, the well-known harp-shells 

 (Harpa). These are strong, broad, ventricose structures, highly coloured, and 

 adorned with numerous curved ribs, running parallel with the outer lip of the 

 aperture. The columellar margin is smooth, and covered with a thin brilliant 

 callus. The animal has an enormous foot, which is not wholly retractile within 



