STRUCTURE OF THE MOLLUSCA. 



195 



emits a fluid of a deep purple colour, which tinges the water 

 around it. 



The Neritina have the columella straight, which renders 

 tlie aperture of the shell semicircular. To this family may 

 be referred the Natica\ of which the structure of the foot 

 is peculiar, it being composed of two disks meeting at an 

 acute angle, and the Neritoi. 



The families which follow are all furnished with a respi- 

 ratory siphon. 



The Buccinina have an oval or elongated shell, with an 

 oval aperture, and a notch at its forepart. The foot is 

 generally of moderate size and rounded before, the mantle 

 entire on the margin, and prolonged anteriorly into a canal, 

 the mouth armed with a proboscis. Of this kind is the genus 

 JBuccinum, from which JVassa differs chiefly in having the 

 columella covered with a thick plate of calcareous matter. 

 The Doha belong to the same tribe, and are remarkable for 

 the length of their proboscis, and the great size of the foot. 

 The Harp(B have the foot abrupt before, very large, and 

 tapering to a point behind. The genera Purpura, Cassis, 

 Terebra, and Cerithium, differing greatly in the form of the 

 shell, and considerably in the structure of the animal, may 

 be considered as part of the same series. 



The Muricina have the aperture of the shell always pro- 

 longed into a canal generally of great length. The animals, 

 however, greatly resemble those of the Buccinina. They 

 have a proboscis, and the right edge of the mantle furnished 

 with lobes or filaments. They are all carnivorous, and in- 

 habit the sea. The Murices are distinguished by their shell, 

 of which the canal is long and straight, and the turns fur- 

 nished with transverse ridges or varices, often presenting 

 spines or lamellae, which are frequently branched or frondose. 

 The Fusi have the canal also long, but they are destitute of 

 varices. The Stromhi have the outer edge of the aperture 

 extremely dilated when old. In the Plerocercc it is divided 

 into long processes. 



The Volutina have the columelhi furnished with oblique 

 plaits. To this tribe belong the Volutes, Mitra:, and OUvte. 



r2 



