MOLLUSCA. 85 
The number of species and varieties being so great in this genus, Lamarck 
was led to doubt the permanency of species, and he judged that those which 
seemed better established in other cases, would present equal uncertainty 
were their number greatly increased. 
Fam. 8. Vermetide. The genus Vermetus (V.lumbricalis, pl. 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the foot (as there is no locomotion) is obsolete, the posterior portion being 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 their base externally. There is a single branchia. 
Cuvier placed this genus, with Magilus and Siliquaria, in a distinct order 
named Zubulibranchia. In the genus Magilus the young has an ordinary 
short ventricose turbinated shell (having a distant resemblance to pl. 75, 
jig. 91). We have seen, in the case of Aspergillwm (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. 
fram. 4. Trochide. 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 (I. solaris, pl. 75, fig. 106; T. magus, fig. 107; Turbo, fig. 108) 
Monodonto and Delphinula (D. delphinus, fig. 104) are nearly allied, and 
the animals do not differ. In Solarium (S. perspectivum, pl. 75, fig. 108), 
the shell and animal differ, the head not being proboscidiform. 
Janthina vanthina (pl. 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. This 
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 Haliotes (H. tuberculata, pl. 75, fig. 86), the shell is ear-shaped, 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 ENCYCLOP £DIA.—VOL. II. 19 289 
