MUREX. 501 
mantle of the animal, and the foot is invariably furnished 
with an operculum. We have shown that the British Plewro- 
tomata are almost always without opercula; the genus has 
scarcely a malacological support; it rests solely on the emar- 
gination in the upper part of the outer hp and the corre- 
sponding sinus of the mantle, which, in the British species, is 
not cloven, as im the true exotic Pleurotomata. These slight 
characters, whether of the shell or the animal, so far from 
bemg essential permanent ones, are most variable and uncer- 
tain, shading off in the numerous species, from the deep 
pleurotomic scission into the simple, scarcely perceptible 
canal of the Murices of our second section, the Fusi of au- 
thors. No one can define the boundary of this arbitrary 
generic index, which in many species does not even indicate 
specific variation. 
Dr. Leach placed them in his genus Mangelia, for what 
reason does not appear, but I can see nothing in those I have 
described to justify the creation of a genus for their animals 
distinct from Murex. I view them as Murices in which the 
opercula have vanished or become obsolete; I have therefore 
placed them as the last section of the genus Murex, consider- 
ing them as on the confines of the family, and forming the 
passage to the exotic genera Cancellaria, Dolium, Harpa, Mitra, 
Voluta, and Conus, all of which, except Conus, which has a 
minute operculum, are without that appendage; and though 
these families are not the typical Canalifera, still it is clear 
that the Columellariade and Convolutide have very many 
points of connection with the Muricide. In this section 
there are two or three British species, the animals of which 
have not occurred to us; amongst them, the Pleurotoma 
teres, nonnull., which is placed here provisionally, being the 
only British species without longitudinal ribs: the animal 
may be the true exotic Plewrotoma with an operculum; the 
character of the scission is peculiar, and more in accordance 
with that genus: its position must remain in doubt until the 
soft parts have been examined. 
We have recently learnt that the M. teres and M. Leufroyi, 
the Boothii of authors, have no operculum, and consequently 
