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NOTES ON THE GROWTH AND VARIATION OF 

 UNIO PICTORUM (Linne). 



By W. E. ALKINS, M.Sc. 



(Read before the Society, May 14th, 1919). 



The statistical study of variation of species seems to offer a prospect 

 of furnishing valuable results from the point of view of a more 

 accurate and close definition of species, although many more data 

 are required before broad generalisations are possible. The more 

 immediate value of such research lies in the fact that it directs atten- 

 tion very definitely to the conception of a species as a group of 

 individuals which approximate, in each and every given character, to 

 a certain mean value, the divergence of the various individuals from 

 this mean being in accordance with the law of chance. 



The following enquiry is based on a series of 250 specimens of 

 Unio pidorum (Linne), collected about thirty years ago, in the Here- 

 ford Waterworks Reservoir, where they lived in fine mud in soft 

 moorland water. 



The dimensions of the shells were determined by means of an 

 optician's sliding gauge, reading in millimetres, and each was taken 

 to the nearest millimetre. The "length" was taken as the greatest 

 dorso-ventral axis, perpendicular to the hinge-line, and was found by 

 placing the fixed arm of the gauge along the ligament, with the valves 

 closed, and bringing up the movable arm until it was just in contact 

 with the ventral margin — of course near the posterior end. The 

 "width" was taken similarly as the greatest antero-posterior axis, 

 parallel with the hinge-line, and the " thickness " as the greatest 

 lateral axis, perpendicular to the length and width axes. The values 

 of the three were noted for each shell, and the ratios width : length 

 and thickness : length were determined. 



As a preliminary investigation of the results obtained, the number 

 of shells of each length, width, and thickness was found, and the 

 corresponding distribution curves plotted ; these are of no particular 

 interest. The length curve had a maximum, at 26 mm., with a sub- 

 maximum at 32 mm., the width curve had two maxima at 55 and 75 

 mm. ; and the thickness curve shewed a maximum at 16 mm., and a 

 sub-maximum at 22 mm. (the units taken for the three curves were 

 2 mm., 5 mm., and 2 mm. respectively). The occurrence of a second 

 maximum or a sub-maximum in each case appears to indicate that 

 the specimens belonged mainly to two generations. 



