132 THE NAUTILUS. 



volving lines whose upper and lower boundaries are well defined. 

 Occasionally the two are merged into one broad band. The 

 majority of the specimens in this collection show traces of this 

 ornamentation and in many the lines are quite clearly marked. 



FACTORS AFFECTING VARIATION. 



As seen from the above paragraph, considerable variation 

 occurs in Planorbis campanulatus in the area under discussion. 

 Although, in the literature, references to deviations from type 

 are not common, this form would seem to be a somewhat plastic 

 species reacting to some unusual external condition. In this 

 case the writer attributes these modifications mainly to bottom 

 environment. The habitat of P. campanulatus is usually given 

 as ponds or streams with a muddy bottom, or weedy areas with a 

 muddy or sandy bottom, presumably in well-protected places. At 

 Blue Sea we have an entirely different set of conditions. This 

 lake is wholly within the Archaean granite and limestone area; 

 its shores, especially toward the north, are precipitous and 

 rocky; and its floor, with the exception of some small muddy 

 bays is composed of bed rock. No streams of any size enter 

 the lake and consequently little sediment is being deposited. 

 The larger of these bays are at the south end, and none of the 

 specimens here discussed were collected there. The shells are 

 often found in from one to three feet of water attached to the 

 rock. Upon these rocks, absolutely devoid of sediment, the 

 waves during a storm beat with great force. Yet, while waves 

 of considerable strength were beating upon the shore, causing 

 small pebbles, which were placed as markers, to be tossed vio- 

 lently to and fro, this gastropod would remain firmly attached 

 by its foot to the rock. This habitat differs vastly from the 

 muddy bottom of rivers or ponds. The animal holds its shell 

 erect. Any increase in obliquity of the aperture causes the shell 

 to be carried more horizontal^, and consequently better adapted 

 to resist wave action. In all probability this environment has 

 developed the high degree of obliquity of the specimens from 

 Blue Sea. Planorbis defiectus is an example of a form which has 

 a somewhat oblique aperture so that it can lie almost flat, and 

 is found occasionally on exposed rocky shores. 



