378 ORDER PECTINIBRANCHIATA. TROCHOIDJE. 



are fixed, occupies the last whorl, or turn, of the shell ( 903) ; 

 and in some of the Order there is a tubular prolongation of the 

 mantle, termed the siphon, for the purpose of conveying water into 

 this cavity, so that the animal can respire without putting forth 

 its body from its shelter. By the presence or absence of this 

 organ, and by the form of the shell (which here appears to bear 

 a sufficiently constant relation with that of the animal), this 

 large group may be arranged under the following families : 



I. TROCHOID^E, or the Periwinkle tribe, in which there is no 

 siphon, and which have the mouth closed by an operculum, 

 usually calcareous. 



II. CAPULOIDJE, which have a wide open shell, very much 

 like that of the Limpets, without an operculum, and destitute of 

 a notch at the margin for the passage of the siphon. 



III. BTJCCINOID.E, or the Whelk tribe, which have a spiral 

 shell, and a canal at the end of the columella for the passage of 

 the siphon. 



919. The family TROCHOID.E is a very extensive one, and 

 includes numerous species which are interesting on account of 

 their peculiar habits, as well as for the beauty of their shells. 

 In the genera Trochus and Turbo, which may be considered its 

 types, the interior of the shell is nacreous; and the exterior, when 

 cleaned, usually presents a very pleasing mixture of colours. The 

 shell, though spirally coiled, usually has a more or less regular 

 conical form (as is seen in the common Periwinkle) ; the first, 

 or earliest whorl, forming the point of the cone, and each whorl 

 extending a little beyond the one above it, so as to increase the 

 diameter of the shell very regularly ; and the mouth being 

 situated at the base of the cone, which in the Trochus is almost 

 flat. The Trochus is distributed all over the world ; and 

 at least seventy species of it are known, the largest being 

 restricted to tropical climates. Among these we may especially 

 notice the Trochus agglutinans, a native of the West Indies, 

 which derives its name from its singular habit of glueing to its 

 shell small pieces of stone, coral, shell, &c. This seems to answer 

 the purpose of strengthening its shell, which is thin and brittle. 

 A fresh-water species from Brazil has been described, in which 



