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REVERSED SHELLS IN THE MANCHESTER MUSEUM. 



By R. STANDEN. 



(Read before the Society, September 13th, 1905). 



The Rev. G. A. Frank Knight, in an interesting discourse delivered 

 to the Perthshire Society of Natural Science, on " Sinistrorsity in the 

 Mollusca," observed that there exists no list of the Mollusca in which 

 the phenomenon of reversal of the shell-spire is known to occur. 

 Although the compilation of such a catalogue would be beyond the 

 powers of a single conchologist, it appeared that the work might be 

 accomplished by co-operation, and as a first instalment I venture to 

 offer the following list of reversed specimens contained in the Man- 

 chester Museum. Perhaps its publication may incite other curators 

 to contribute information on the same topic from the collections under 

 their charge, and thus by degrees the list will approach completeness. 



By way of introduction to the subject, I quote, with Mr. Knight's 

 permission, a few paragraphs from his lecture, explanatory of the 

 subject of " Sinistrorsity," or reversal of the shell, which I find the 

 majority of persons have a difficulty in understanding : — 



"A univalve spiral shell is said to be 'dextral' when the mouth 

 opens to the right hand of the observer, as he holds it with the spire 

 pointing upwards. A univalve spiral shell is said to be 'sinistral' 

 when the aperture is to the left hand of the observer, as he holds the 

 shell with its apex upwards. If we imagine the interior of the shell 

 to be a spiral staircase, then, as we ascend a dextral mollusc, the 

 'axis' or 'columella' of the stair would always be at our left hand, 

 and similarly, if the mollusc be sinistral^ the stair up into its interior 

 would always curve round the axis on the right hand. 



" The whole subject of convolution in the mollusca is one of 

 extreme interest, and has excited the enquiries of eminent scientists. 

 As far back as 1838, we find the Rev. Canon Moseley contributing 

 an elaborate essay 'On the Geometrical Form of Turbinated and 

 Discoid Shells,' which was published in the Philosophical Transactions 

 for that year. In this paper he proves that the laws which determine 

 even such an apparently insignificant matter as the mode in which 

 the shells of univalves are spirally twisted, are as mathematically true 

 as the conic sections which regulate the orbits of the planets and 

 comets ! He show^s that the size of the whorls, and the distance 

 between contiguous whorls, in such shells as the common Turritella 

 of our shores or Phuiorbis of our ponds, follow a geometrical 

 progression. The spiral formed is the "logarithmic," of which it 

 is a i)rupcrty that it has everywhere the same geometrical curvatuie, 

 and is the only curve, e.\cept the circle, which possesses this property. 



