84 
NATURAL HISTORY. [NEW BUILDING, 
Purpura Lapillus yields a beautiful purple colour, which 
has been considered as the Tyrian dye of the ancients ; but 
there is little doubt that this colour was obtained from 
various species, which were most common in the district; 
the true Tyrian dye was most probably derived from 
Murex trunculus. 
The family of Strombidce (Cases 1, 2) are peculiar for 
having a very compressed foot, which only allows them to 
move from place to place by putting their foot across the 
line they wish to progress, and then turning themselves 
over towards the places of their destination, and con- 
tinually repeating this progress; they are active ; their 
eyes are large, and placed on the end of a large thick 
elongated peduncle, having the slender tentacles arising 
out of the middle of the hinder sides, and the shell is 
peculiar for having a deep sinus placed on the side near 
the canal over the head of the animal, when it is expanded. 
These animals, when they arrive at their full size, expand 
the edge of the mantle in a remarkable degree, which 
causes the shell of the adult animal to be very different 
from that of the young. This expansion only takes place 
once in their lives, and not at repeated and stated periods, 
as in the Murices , &e. Their operculum is narrow and 
claw-like, the apex being produced beyond the point of 
its attachment, as it enlarges, by new layers below,ifhey 
i use it as a weapon of offence when they have turned them- 
I selves on their back, and the tube of the syphon is al¬ 
ways bent towards the right side. They live on dead flesh. 
The genus Terebellum have no operculum, and live in 
muddy places in deep water. 
All* the other flesh-eatirg comb-gilled Gasteropodes have 
a broad expanded foot, by which they glide on and attach 
themselves to marine bodies; their eyes are sessile, or 
placed on a very short tubercle near the base, or on the 
tentacles. 
The family of Murices ( Muricidce , Cases 3—8) have a 
more or less elongated straight syphon, and the shell has a 
straight tubular canal for its protection ; the animal, at cer¬ 
tain periods of its growth, expands the edge of its mantle, 
and, during this time, deposits appendages on the edge of 
the shell for their protection; these expansions of the man¬ 
tle are then gradually withdrawn, and the portion of shell 
