f^at^i.Jky( ^p. I'^^' 



135 



TURTHER OBSERVATIONS ON THE OCCURRENCE 



OF MANGANESE IN LAND AND FRESH -WATER 



MOLLUSCA. 



A. E. BOYCOTT. 



Since the publication of the previous paper* I have been able 

 to examine a considerable amount of fresh material, much of 

 which I owe to the kind help of various correspondents, and 

 especially of the late W. D. Roebuck. The number of species 

 examined has been increased from 56 to 79, and much further 

 information has been obtained about the occurrence of man- 

 ganese in these already dealt with. 



The data are briefly summarised in the table which shows 

 the parts of manganese per 10,000 parts of dried snail body ; 

 ' trace ' means less than o'l. 



A. (a). — The helicids — 18 species from P. rotundata to H. 



hortensis — have relatively little manganese. In 

 130 analyses from 122 localities a result of more 

 than 5t occurred only once {H. lapicida from the 

 Forest of Dean) and in 85 instances it was less than 

 one. 



(&). — The small bivalves, Sphaerium and Pisidium, also 

 have very little, the highest figure in 11 analyses 

 of the five species being o'3. 



(c). — The operculates, land and freshwater, similarly 

 give low results, only once exceeding 2 in 17 

 analyses. 



B. {a). — Contrariwise, the large bivalves, Unio, Anodon, 



Psetidanodonia and Margaritana, uniformly contain 

 large amounts, J 33 analyses from 22 localities 

 ranging from 18 to 212. 

 (&). — Bnliminns also ranges from 30 to 103 in 16 analyses, 

 and Zonitoides excavatus gives the highest average. 



. (^73). 

 C Clausilia, Siiccinea, the aquatic pulmonates, Dreissena 

 polymorpha, Vitrina pellucida and Zonitoides 

 nitidus give moderate and rather variable figures. 

 D. (a^).— The slugs, as a whole, give much more variable 

 results than the conchifers, and the mean figures 

 may be high or low. 



(i). — Testacella is usually quite low, though a 

 sctitulum from Berkhamsted gives 39. 



* The Naturalist, 1917, pp. n, 69. 



t i,e., parts manganese per 10,000 parts dried snail body. 

 X As originally described by H. C. Bradley, Journ. B^l. Chem., 

 III. (1907), 151 ; VIII. (1910), 237. 



1921 April 1 



