SHELL GALLERY. 57 



The collection is contained in four parallel rows of table- 

 cases, the arrangement commencing on the right as the gallery 

 is entered. The first two rows contain the marine forms of 

 Gastropods — a division which includes Snails, Slugs, Whelks 

 (fig. 28), and aU those Molluscs which crawl upon the under 

 surface of their bodies ; the Cones, Volutes, Mitras, and Murexes 

 forming some of the most attractive groups. The two rows of cases 

 on the left contain the Land-Shells, Bivalves, and Cephalopods. 

 The Cockles, Oysters, Clams, Piddocks, Teredos, Scallops, and 

 Ark-SheUs represent some of the principal types of Bivalves, 

 so called on account of their shells being formed of two pieces 

 or valves. A Giant Clam (fig. 29), on the floor of the gallery. 



y 



Fia. 30.— The Peaelt Nautilus (Nautilus pompilim). 



a, body ; 6, siphuncle ; c, eye ; d, hood ; s, tentacles ; /, muBcle of attachment 



to the shell ; g, siphon. 



weighs 310 lbs. The Argonaut, the beautiful Pearly Nautilus 

 (fig. 30), the Octopus, Squids, and Cuttlefishes are the chief kinds 

 of modern Cephalopods ; but there are a number of fossil forms, 

 most of which are exhibited in the Geolog'ical Department. 



In the same gallery are exhibited three other groups of marine sea-llats, 

 organisms, respectively known as Polyzoa, Brachiopoda, and ^nd'sel^*^' 

 Tunicata. Many of the former live in colonies, and are squirts, 

 known as Sea-Mats. They are often mistaken for sea-weeds, 

 although they are really animals of high organisation. The 

 Brachiopods have two shells like bivalve Molluscs, only the 

 valves are dorsal and ventral — that is to say, back and front 

 — instead of right and left. One valve is frequently per- 

 forated,- hence the name of "Lamp-shells." Sea-squirts, 



