(LECUM. 325 



sutures are very different from the whitish transverse lines 

 that are sometimes visible in the Truncatella atomus and 

 R. planorbis. 



However, as we have doubts of the special individuality 

 of the ? Skenea rota, we deposit here this apocryphal object, 

 with references : 



S. 1 rota, Brit. Moll. iii. p. 160, pi. 73. f. 10, and pi. 88. f. 1, 2. 



The anatomy and the external organs of the Vermeti and 

 C(Eca closely agree. We have already mentioned, that the 

 incidents of the shells mainly correspond. Notwithstanding 

 these affinities, there are perhaps sufficient characters to sepa- 

 rate the genera. Therefore, for the present, we will consider 

 Ctecum so aberrant from Vermetus as to require a distinct 

 genus. 



The Vermeti are generally, perhaps always when young, 

 attached to marine substances. The Caeca, when adult, are 

 certainly free, but it is not improbable they may be fixed in 

 the embryonic condition ; indeed there is reason to think that 

 this idea may be well founded ; the excessive rarity of C. gla- 

 brum in the adolescent state, in which it has the apical coil of 

 2^ volutions, supports this view. The so-called Skenea rota 

 is equally rare, and also corroborates my opinion that the 

 Caeca, when young, are fixed, and at a certain age become 

 free, that is, on the formation of the first plug ; and we may 

 observe, that before complete maturity a succession of plugs 

 are formed as the animal withdraws from portions of the 

 cylinder, which wither and become decollated ; and thus the 

 original spiral coil remains attached to the spot on which it 

 was first cast, and in consequence is rarely met with. In 

 addition we observe, that in the C. trachea the periodic pauses 

 in the growth are as well marked as in the Vermeti : there 

 are often four equidistant, well-defined rings, some of which 

 have a slight tendency to intus-susception. 



The Vermeti have been described as possessing four tenta- 

 cida, but two of them are nothing more than the well-de- 

 veloped pedicles of the eyes, like those of the Trochi ; and the 

 foot is without an anterior production, the posterior portion 



