Shell of Fresh-water Mussel. 57 



known as the ' pallial line/ and which corresponded in the living 

 animal to the line along* which the muscular border of the mantle 

 was more or less loosely attached, just where it became confluent 

 with the central lobes of the organ, describes a curve, similar to 

 those of the concentrically arranged ribs on the outside of the 

 shell, between the two principal muscular depressions. Certain 

 smaller muscular depressions are visible just anteriorly to the apices 

 of the umbones, marking the points of origin of certain retractor 

 fibres. The thinner, lighter coloured, and less eroded of these two 

 shells came from shallow water ; the thicker, darker, and more 

 eroded is an ordinary deep water form. That the amount of in- 

 organic salts which these as also other organisms take up, is by 

 no means dependent upon the amount present in the medium in 

 which they live, but upon the selective working of their tissues, 

 which may be intensified or diminished by other chemical and also 

 by non-chemical conditions, is seen from instances such as these, 

 as also from a comparison of the dense shell of the Pearl Mussel 

 from mountain streams, such as those of Westmoreland, with the 

 thin light shells of the Anodons from the Oxford waters so much 

 richer in salts of lime. 



For an excellent disquisition upon the last-mentioned point, as well 

 as upon many others relating to the physiology of the Pearl 

 Mussel, Unio Margaritifer, a species allied to the Anodon, 

 see Voit in Siebold and Kolliker's Zeitschrift fur Wissen- 

 schaftliche Zoologie, x., 1859, p. 470. 



For a monograph of the former species, see T. von Hessling, Die 

 Perlmuscheln und ihre Perlen, Leipzig, 1859. 



For accounts of the formation and structure of the shell, see Bronn, 

 Klassen und Ordnungen des Thierreichs, iii. i. p. 420 ; Voit, 

 1. c. p. 487 ; Huxley, Cyclopaedia of Anatomy and Physiology, 

 Article "^ Tegumentary Organs;' Carpenter, British Assoc. 



Reports, 1847, P^- i" .» %^- ^' 9' '°- 



For a description of the animaFs muscles, the impressions of which 

 on the interior of the shell are mentioned above, see Poli 

 Testacea Utriusque Siciliae, torn. i. p. '3^6, tab. ix., fig. 2. 



For the amorphous non-crystalline character of the calcareous de- 

 posit in the shell of Unio, see Hesshng, 1. c. pp. 251, 261. 

 See also Rose, Berlin Abhandlungcn for 1858, p. 98. 



