CCUM. 325 
sutures are very different from the whitish transverse lnes 
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.? rota, Brit. Moll. ii. p. 160, pl. 73. f. 10, and pl. 88. f. 1, 2. 
The anatomy and the external organs of the Vermeti and 
Ceca 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 
Cecum so aberrant from Vermetus as to require a distinct 
genus. 
The Vermeti are generally, perhaps always when young, 
attached to marine substances. The Ceca, 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 
24 volutions, supports this view. The so-called Skenea rota 
is equally rare, and also corroborates my opimion that the 
Ceca, 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- 
cula, 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 
