324 VERMETID^. 



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 strise of growth, which at first view resemble those 

 of the ? 'rota'; but a more careful inspection 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 

 being quite smooth, but on the formation of the first plug 

 and incipient prolongation, it shows, under a common lens, 

 decided annular strise, 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 



