324 VERMETIDZ. 
Skenea rota, and in both, the spire is more depressed on one 
side than on the other. I may be wrong in this conjecture, 
but, arguing from analogy, that as C. glabrum in the young 
state has, beyond doubt, a posterior spiral terminus, I may 
reasonably assume that C. trachea, its congener, has also, 
when young, a spirated extremity ; and from the aspect, form, 
aperture, markings, and general configuration, I shall not be 
surprised to find that the Skenea rota of authors is its spiral 
posterior terminal portion. I have never taken it except in 
the deepest waters of the coral district; but some concho- 
logists say that it has been found amongst the littoral Corallina 
officinalis. I think this is an error, and that their object is 
either the Truncatella atomus of Philippi, the Skenea ? nitidis- 
sima of the ‘ British Mollusca,’ or a dwarf Skenea planorbis of 
the same work; with us, Rissoa planorbis. Both these are 
usually smooth ; still there are frequent examples with strong, 
transverse striz of growth, which at first view resemble those 
of the ? ‘rota’; but a more careful imspection shows that they 
are not of the same shape, and the contour and figure of the 
disk have a different character. We think the learned authors 
of the ‘ British Mollusca’ have properly separated the two. 
it may be objected that the radiating ribs of this minute 
shell do not accord with the comparatively smooth cylinder 
of the very young ‘trachea, nor with the more advanced 
stages ; but it must be borne in mind, that this object is in its 
sub-embryonic condition, that it instantly changes its character 
on commencing the elongation, and passes through four or 
five other diverse phases, until its adult state is completed. 
In corroboration of this view we state, that its congener, the 
C. glabrum, exhibits similar discrepancies. Of that species 
we possess the spiral portion united to the elongation, and 
also without it; the united portions are dissimilar, the spire 
bemg quite smooth, but on the formation of the first plug 
and incipient prolongation, it shows, under a common lens, 
decided annular strize, and displays, until adult, alternating 
spaces of smooth and striated surface. In our specimen of 
the object ?‘rota, there are the same intus-susceptive join- 
ings and divisions as are seen in every C. trachea. 'These 
