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MOLLUSCS. 



the tritons, but they are broader, more globose, and have shorter spires. The 

 aperture is more or less dentate on each side, and the siphonal canal short, and 

 sharply recurved. Upon the columellar side the animal deposits a strong shelly 

 callosity, which in some species is enormous, and unites with the outer lip above. 

 The animal closely resembles that of Triton ; its dentition is similar, but the 

 opercula in this family are peculiar, having a lateral nucleus, and often being 

 rayed like an expanded fan. The shells of some species consist of different- 

 coloured layers, and are made use of for carving shell-cameos, in order that the 

 subject may stand out in relief upon a differently-coloured ground. This family 

 includes the genus Cassis and its subgenera, and the genera Morio and Lambidium. 

 Nearly all the species are from hot regions, but* a few occur in Japan and New 



TUN-SHELL, Dolium perdix (\ nat. size). 



Zealand ; and the well-known Mediterranean Morio tyrrhenus has of late years 

 been dredged living in deep water off' the south-west of Ireland. 



The tun-shells (Doliidce) are mostly thinnish and of globose form, sometimes 

 of very large size, and always spirally ribbed and grooved. They have no varices, 

 and are without opercula. The foot of the animal is large, and the retractile 

 proboscis long, and furnished with an expanded disc at the end, as in the figured 

 Dolium perdix. The shells of the subgenus Medea, which have the outer and inner 

 lips strongly dentate, form a connecting link with the Cassididce. Nearly all the 

 species, about fifteen in number, are tropical. Two, however, occur in the 

 Mediterranean, and several in Japan ; Dolium c/alea, which occurs off the south 

 of France and other parts of the Mediterranean, has a shell 8 to 10 inches in 

 length, and is the largest gastropod of that region. This mollusc, as well as 

 various species of Cassididce and Tritonidce, are said to secrete sulphuric acid. 



