TURBO. 



set volutions, some of which have their columella plain, 

 without any tooth near the base; these are the Lamarckian 

 Trochi: others have a more or less distinct tooth near the 

 base of the columella, these are Lamarck's Monodonta.^ 

 Delphinula appears to us to be distinguished by its circular 

 aperture and its thickened and reflected peritreme. The 

 operculum of Littorina is spiral, its volutions are, however, 

 very few and increase very rapidly in width, its columella 

 is moreover in general somewhat depressed. 



It is perhaps incumbent on us to notice two endeavours 

 to clear up these genera in part, by separating some 

 shells from them under the names of Margarita and 

 M armarostoma, the first of these has been given to some 

 small thin shells whose operculum is horny, and spiral with 

 numerous closely-set volutions, and belongs to our Trochi; 

 the latter has been given by Swainson to a typical species 

 of Turbo. The Turbines vary much in general form, some 

 being conical, with a very flat base, and keeled edge, and 

 others having a very regular top-shape. They are in 

 general thick, with a spire of moderate length, though 

 sometimes very short ; their aperture is for the most part 

 nearly circular, though sometimes rather transverse and 

 somewhat trapezoidal ; their peritreme rather sharp and 

 not reflected, somewhat spreading at the lower part; 

 operculum shelly, solid, externally very various in its 

 appearance, but internally covered with a spiral horny 

 plate ; we think it the more necessary to mention this 

 latter circumstance as many have mistaken the rounded 

 external surface of these opercula for the part which is 

 attached to the animal and vice versa. It is observable that 



♦ We have been under the necessity of entirely omitting Lamarck's 

 Monodonta, because upon careful examination we find it merges into Trochus; 

 the operculum is exactly similar in both, and it is observable, that Lamarck has 

 separated them in the most arbitrary manner, without appearing- to have 

 been himself aware of any definite character by which they might be distin- 

 guished ; thus we find him at one time uniting the Linnean Trochus Pharaonis 

 to his Monodonta and afterwards restoring it to Trochus; this it is well known 

 has a very distinct tooth near the base of the columella and should therefore 

 have formed a part of his Monodonta ; and so for the same reason, should 

 Trochus niloticus and his Turbo Pica. We regard the whole of these as legiti- 

 mate Trochi, inasmuch as the operculum in all is precisely similar and quite 

 different from that of Turbo. Some of Lamarck's Monodonta belong to 

 Littorina, and one of them is a land shell, probably related to Auricula, 



