MOLLUSCA. 85 



The number of species and varieties being so great in this gemis, Lamarck 

 was led to doubt the permanency of species, and he judged that those wliich 

 seemed better established in other cases, would present equal uncertainty 

 were their number greatly increased. 



Fam. 3. Vermetida'. The genus Vermetus ( V. luinhricalis^ jpl. 75, fig. 

 69) has a loosely and irregularly coiled shell affixed by its posterior 

 extremity. As the animal enlarges it increases the size of the shell, and 

 moves forward in it, cutting off the empty posterior portion by a diaphragm 

 from time to time. The animal is much like that of Turbo or Delphinula, 

 the foot (as there is no locomotion) is obsolete, the posterior portion being 

 adapted to support a thin operculum which closes the aperture. There are two 

 tentacles, with the eyes at tlieir base externally. There is a single branchia. 

 Cuvier placed this genus, with Magilus and Siliquaria, in a distinct order 

 named TuhuUhranchia. In the genus Magilus the young has an ordinary 

 short ventricose turbinated shell (having a distant resemblance to pi. 75, 

 fi^. 91). We have seen, in the case of Aspergillum (p. 60), how a bivalve 

 shell can take the form of a tubular one ; and Magilus is an example of the 

 same thing in a spirivalve one. The animal inhabits cavities in living coral, 

 and to prevent being buried by the growth of the surrounding material, it has 

 the power of forming a tube, the margin of which it builds up as the coral 

 increases, so that the aperture retains the level of the general surface. The 

 tube thus attains five or six times the length of the original shell, and it assumes 

 various curves and irregularities, depending upon the growth of the coral. 



Fam. 4. Trochidm. The members of this family are herbivorous, and 

 most of them have the mantle or foot ornamented with tentacular appendages. 

 The shell of Trochus is short and conical, solid, and nacreous. The genera 

 Trochus {T. solaris^ pi. 1^., fig. 106 ; T. magus., fig. 107; Turho.,fig. 103) 

 Monodonto and Delphinula {D. delpJmius.,fi,g. 104) are nearly allied, and 

 the animals do not differ. In Solarium {S. perspectivum^ pi. 75, fig. 108), 

 the shell and animal differ, the head not being proboscidiform. 



Janthina ianthina {pi. 75, fig. 96) is the type of a sub-family, distin- 

 guished by the possession of an apparatus which enables the animal to float 

 at the surface of the sea. The name is derived from the Greek word for 

 ■violet, the shells of all the species being of this color. The shell is trochoidal 

 and very fragile, having the right side of the aperture sharp, and often 

 notched. The animal has a large proboscidiform head, two tentacles, and 

 eyes ; the mantle with an expansion said to be used in swimming ; the foot 

 with an appendage or float formed of a great number of air-vesicles. Thi;<- 

 appendage can be cast off and renewed. The latter process was observed 

 by Dr. Reynell Coates, who describes it as being formed by inclosing a 

 bubble of air in a cavity formed by contracting the margin of the foot, 

 which then secretes a covering for it. The eggs are attached to the under 

 surface of the float, and subsequently cast off' with it. This animal was 

 first described and figured by Fabius Columna, in 1616. 



In Ilaliotis {II. tuber culata, pi. 15, fig. 86), the shell is ear-sliaped, much 

 depressed, very short and flat, the aperture longer than wide, and as large 

 as the base of the shell, left side with a sub-marginal row of perforations ; 



ICONOGRAPHIC ENCYCLOPEDIA. VOL. II. 19 289 



