TKOCHUS. 317 



position, we cannot always arrive at facts with certainty. 

 My own impression is that all of them have four vibracula ; 

 but, however this may be, in a generic point of view it is 

 of no moment. Operculum as in T. Cutlerianus. This is the 

 minutest animal of the three, and by far the most active ; 

 thus again showing, as I formerly observed in the f Annals/ 

 on Caecum glabrum, that nature, as she diminishes in volume, 

 usually accompanies that condition with an equivalent of 

 increased energy and activity. 



The remaining species of this section that have not been 

 examined are 



T. UNDULATUS, Sowerby. 



T. undulatus, Brit. Moll. ii. p. 528, pi. 68. f. 1, 2, and pi. 73. f. 5, 6. 

 This is the T. carneus of Lowe. 



T. HELICINUS, O. Fabricius. 



T. helicinus, Brit. Moll. ii. p. 531, pi. 68. f. 4, 5, and pi. 74. f. 10; 

 (animal) pi. C.C. f. 4. 



T. SUBCARINATUS ? Mont, et nobis. 

 Helix subcarinatus, Mont, et aliorum. 

 Adeorbis subcarinatus, Brit. Moll. ii. p. 541, pi. 68. f. 6, 7, 8. 



We think this species, when the animal is discovered, 

 which has hitherto evaded every research, will prove to be 

 a Trochus. 



We believe that the ? Skenea costulata of the ' British 

 Mollusca ' is only a worn T. Cutlerianus, with the spiral lines 

 rubbed smooth or faintly impressed and scarcely visible, which 

 in live specimens is often the case ; the ribs across the volutions 

 are only the bifurcating striae thickened by the action of the 

 atmosphere ; these striae prevail more or less in every speci- 

 men. If the figures of the two in the ( British Mollusca J be 

 examined, their identity will be apparent. 



If it be objected that the preceding descriptions are weari- 

 some, I admit that they are so, as a consecutive whole ; but it 

 is not fair thus to look at the question ; each object must be 



