248 JOURNAI. OF CONCHOLOGY, VOL. I5, NO. 8, JANUARY, I918. 



tinual disturbance from wind, from birds and from those wtio take 

 their pleasure in boats and the expectation of fish. 



Muddiness may affect mollusca in several ways. A gross degree 

 may be reasonably expected to choke them mechanically, especially 

 in respect of their respiration and chiefly if respiration is branchial. 

 Such a degree is, however, the exception in the Aldenham ponds, and 

 I suspect that the main effect of the opacity which is commonly pre- 

 sent in the closed ponds is due to the cutting off of light. If so, 

 mollusca are presumably affected indirectly, through the influence of 

 defective lighting on the vegetation. Muddy ponds show generally 

 few or none of the ordinary water plants ; the plants which are of 

 immediate moment to snails as food are probably algae and suchlike, 

 many of which are epiphytic on the higher plants and which are in 

 any case similarly affected by light. Moreover the absence of ade- 

 quate illumination means that the plants will not perform their 

 function of oxygenating the water. How little muddiness is necessary 

 to obstruct the growth of plants is well shown if two jars are sown 

 with fragments of e.g. Eiodea and the one rendered slightly perman- 

 ently muddy by the addition of a little clay shaken up in water ; in a 

 few weeks the plant in the muddy water has wilted away, in the other 

 has shown active growth. 



Of the 27 closed ponds which contain no snails, in five "muddy" 

 is noted as the presumptive cause. The influence of lighting is also 

 illustrated by the fact that ten more are entered as "shaded by trees," 

 and four others as "partly shaded." 



(3). Closed ponds are further disadvantageously placed as regards 

 foulness. The exaggerated natural effect is best seen in ponds in 

 woods with the floor covered with "trash," as the American limno- 

 logists call it, of dead leaves, twigs, etc. Such ponds contain neither 

 plants nor snails. The absence of plants means deficiency of oxygen, 

 and such decomposition as goes on will be on anaerobic lines, pro- 

 ducing various foul-smelling and probably more or less acid and 

 poisonous substances from dead organic matter. With free oxygena- 

 tion decomposition is more rapid and complete, and leads to bene- 

 ficial results in the way of the production of plant food. But even 

 with the most ample illumination, running ponds and rivers must 

 have the advantage over closed ponds in the way that "trash" is 

 mechanically removed and, especially in winter, the whole water area 

 is Hable to be scoured out with floods. Foul mud is commoner than 

 foul water, so that the point may have special reference to bivalves, 

 particularly Anodonta and Ufiio in their early stages. 



(4). As regards ^i?^, closed ponds are at an obvious disadvantage 

 owing to their limited collecting grounds. Any ordinary small pond 



