16 THE NAUTILUS. 



Branford River has every individual a deep brown. I wonder 

 if this is due to the presence of a large iron foundr}-- a short dis- 

 tance away. Erosion is not a characteristic, 3'et it does result 

 from environment. This is conspicuous where unios are taken 

 from waters where there is decaying vegetation. The reason is 

 well known ; acid from decaying leaves acts on the lime of the 

 shell. The same effect may be seen in marine shells taken from 

 waters where a river meets the sea. An interesting effect of air 

 may be seen in the author's cabinet. Shells of Fusus decem- 

 costatus taken below low tide are well preserved even to the 

 apex ; others from a few j'ards awaj'^, exposed to the air at low 

 tide, are much eroded. 



Pure white sets of Gemma gemma may be found at Woods 

 Hole and Branford, Conn. At Provincetown they are a very 

 dark purple, some specimens showing hardl}' a trace of white. 

 At Revere Beach the type is white with perhaps a third of the 

 shell faintl)^ tinted purple. Reasons for this variation I cannot 

 give. Nor can I explain wh}- sets found a dozen years ago at 

 Revere Beach should vary from specimens obtained at the same 

 spot a 3^ear ago. 



Litorinella minida is abundant in pot holes in the marshes. 

 Environment there is good for it. A dwarfed variety is found 

 in the Branford River, and the same form occurs on dead eel 

 grass under the wharves at Provincetown. Evidently this form 

 flourishes better in still water. Temperature has its influence. 

 Planorbis trivolvis occurs throughout New England. It is 

 small in the Connecticut River at Springfield, the same at Bran- 

 ford and eastern Massachusetts. In northern Maine large, and 

 the largest and finest set in the author's cabinet was obtained at 

 Dalton, Mass., in the Berkshire hills. This last region is well 

 known as having northern insects, undoubtedly the other species 

 of shells would show aflfinitj' with the forms in northern New 

 England. Climate alone explains this variation. The forms 

 from the Connecticut River certainly have water enough, but 

 they are small. Other sets are from small bodies of water; only 

 in colder regions does this giant thrive. 



Environment drives out some species and retains others. 

 These few examples are variations in the same species which 

 shows the eff'ects of different surroundings. 



