MUREX. 501 



mantle of the animal, and the foot is invariably furnished 

 with an operculum. We have shown that the British Pleuro- 

 tomata are almost always without opercula; the genus has 

 scarcely a malacological support j it rests solely on the emar- 

 gination in the upper part of the outer lip and the corre- 

 sponding sinus of the mantle, which, in the British species, is 

 not cloven, as in the true exotic Pleurotomata. These slight 

 characters, whether of the shell or the animal, so far from 

 being essential permanent ones, are most variable and uncer- 

 tain, shading off in the numerous species, from the deep 

 pleurotoinic 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, 

 Valuta, 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 Columellariada and Convolutida have very many 

 points of connection with the Muricidce. 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 Pleurotoma 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 



