352 



LOWER INVERTEBRATES. 



manipulation necessary in this work can only be acquired by experience ; the general 

 shape must first be wrought out, care being taken to leave every projection rather in 

 excess, to be gradually reduced as the details and finish of the work are approached." 



About fifty species of helmet shells are known, all from the warmer seas. Cassis, 

 the largest genus, has its metropolis in the eastern seas, a few species being found in the 

 West Indies and in the Mediterranean. The species most used for cameos is the black 

 helmet, C. madagascarensis, in which the white outer layer covers a darker, almost 

 black, second layer. The external surface is not, as the common name would appar- 

 ently imply, wholly black. The whorls are a dirty white, and the outei- part of the lips 

 are rosy, the black being at and near the edges of the aperture. It reaches a length 

 of nearly a foot. C. glwuca, the species figured, is smaller ; it comes from the East 

 Indies. The other genera of the family .are Cassidaria, Oniscia, and Pachyhatron. 



Fig. 461. — Dolium perdix, tun shell. 



The members of the family Dolud^ are l.irge, with thin shells, the whorls of which 

 are ventricose, the body whorl being very large, and ornamented with revolving ribs. 

 The animal is large, and is provided witli a remarkably developed proboscis, which is 

 long, cylindrical and flexible. The foot is very large and lobed in front ; the tentacles 

 arise from a distinct head, and the eyes are placed on small pedicels growing out from 

 the bases of the tentacles. In the young the shell is closed by an operculum, but it 

 disappears with growth. 



The species of Dolium are more or less globular, with a very large aperture, and 

 have received the common name, tun shells, while the species of Ficida are known 

 from their shape as fig or pear shells. Both genera .are ti-opical or sub-tropical, active 

 molluscs, living on animal food. The shells are not usually brightly colored, but the 

 animals themselves are handsomely marked. 



Last in the order comes the Teitonid^. In conchological characters it belongs 

 near the Muricidae, but in dentition, development etc., its place is here, the lingual 



