480 MURICIDA. 
which the eyes are placed externally ; from thence the tenta- 
cula taper conically to their extremities. The mouth is a 
vertical fissure beneath the tentacular veil, and emits the 
characteristic proboscis. The foot when quiescent is nearly 
oval, but on the march is truncate in front, throwing off on 
the right and left small auricular points; it is gently con- 
stricted medially, and has a blunt rounded termination, 
carrying on the posterior upper surface an elongated, red- 
brown, corneous, unguiculated operculum. 
Lamarck’s commentators say, that between the genera 
Murex and Purpura there are sufficient marks of distinction, 
and, in support of this opinion, they adduce the truncation of 
the tentacula at their offsets in Purpura, which they state is 
more apparent than in Murex. We dissent from these views, 
and think the distinction is purely ideal—at least it is so in 
the species of each genus we have examined: the fact is, that 
when the tentacula are collapsed, the basal two-thirds appear 
very tumid and broad at the termini of the offsets, but in fully 
extended action the truncation nearly or altogether vanishes, 
and no peculiarity is apparent at these points. 
It will be observed below, that the external organs of 
Murex lapillus, the Purpura of authors, are nearly identical 
with those of M. erinaceus, and the internal organs of the 
two are so similar, that it would be a repetition to describe 
them. The corneous opercula scarcely show distinction; that 
of this species may be generally of a deeper red, and somewhat 
rounder, though the arches of the striz are not less elliptical. 
The gland producing the purple dye is as conspicuous as in 
M. lapillus; indeed this gland may be traced in all the 
Canalifera, though its secretion varies in colour; there may 
also be a slight difference in the lhngual riband, which is here 
rather longer and more coiled than in its congener, and the 
cerebral ganglia are smaller: but these variations are of little 
value. The different hues of brown in Murex erinaceus form 
the ground colour, but are invariably mixed with white or 
flaky-yellow markings on the upper part of the foot and on the 
tumid portion of the tentacula, the conically-pomted upper 
parts being of a uniform colour; the under part of the foot is 
