AI.KINS : GKiiUTH AND VAKIAl ION OK UNIO PICTORUM. 233 



Summary anu Conclusion. 



From measurement of the length, widtli, and thickness of a series 

 of Unio pictoriDH it has been shewn that : — 



I. — The width is directly proportional to the length throughout 

 growth. This is true also of the ontogeny of individual shells. 



2. — The thickness (T) is related to the length (L) according to the 

 law : — 



i'66 + o'o.f4L 



T = e 



over the range L = 23 to L = 35 mm. 



Literature.- 



I. — Une Loi Fondamentale de I'Accroissenient des Organismes, 

 par Chr. Petersen. Copenhague : Imprimerie Bianco Luno ; 1919. 



Pearl in Littorina littorea L. — In the course of consumption a small pearl 

 was found in a specimen of [.ittorina littorea. It is quite spherical, and of a red- 

 dish colour, and i '5 mm. in diameter. The only other instance I have seen 

 recorded is by Jeffreys, Brit. Conch., vol. iii, p. 373, where the peatl was one- 

 tenth-of-an-inch, or about 2"5 mm. in size, and was "round and white.'' — W. T. 

 Elliot r (Read before the Society, February 2nd, 1921). 



Pisidium lilljeborgii in Montgomeryshire. — In October, 1920, I collected 

 several specimens of P. lilljebor<:;ii in Llyn Du, a tarn at about 650 feet near Meifod 

 in the valley of the Vyrnwy. They were living in silt at the roots of Isoetes at a 

 spot where the bed of the tarn was stony, and associated with them were P. milium 

 and P. hibernicunt in small numbers. Near the outlet of the tarn there was a good 

 deal of mud, and here P. milium, P. subtrnncatum, P. nitidum, and P. caser- 

 lannm occurred in some plenty. The only other molluscs I noticed were Limnaa 

 pereger, Plaiiorbis albus, and Valvala piscinalis. — Charles Oldham {Read before 

 the Society, December 8th, 1920). 



PROCEEDINGS OF THE 

 CONCHOLOGICAL SOCIETY OF GREAT BRITAIN & IRELAND. 



498th Meeting, held at the Manchester Museum, January 5th, 1921. 



Mr. G. C. Spence in the chair. 



Donation to Library. 



" Les Variations et leur Ileredite chez les MoUusques," by Paul Pelseneer 

 (presented by the author ). 



