No. 5-] THE INTRODUCED LITTORINA. 253 



of the marine fauna and flora in these two localities are appar- 

 ently the same, and the Newport curve is not materially unlike 

 the Seaconnet curve, though its amplitude of variation, owing 

 to the lack of individuals presenting extreme elongation, is two 

 degrees less. The ventricose shells are about evenly divided 

 in the two localities. 



Charts XI and XII.-- Bristol Narrows lies at the mouth of the 

 Kickemuet River, about seventeen miles north of Newport. The 

 water, though sufficiently salt to enable starfish to flourish, is 

 of somewhat less specific gravity than at Newport or Seaconnet. 

 Two sets of shells were examined. One complement of 1000 

 was taken at a shingle beach, from among stones ranging from 

 the size of one's fist to that of one's head ; the other series 

 was collected at a spot only a few hundred feet distant, where 

 the animals were living upon the sand and mud. The two 

 curves are remarkably alike; they are located at the same place 

 on the base line, between the ordinates of 83 and 101, and thus 

 have the same amplitude of variation; vis., 18. 



Chart XIII. - - The shells tabulated on this chart were col- 

 lected at the mouth of the Warren River at a point about three 

 miles from Bristol Narrows. They form a perfectly typical 

 American curve with abrupt ascent, low summit, and sweeping 

 descent. The amplitude of variation is 20, and the curve lies 

 between the ordinates of 84 and 104. 



Thus we have examined shells from ten American localities, 

 and in every case we have found that their amplitude of varia- 

 tion is in excess of the most variable British shells, and we 

 conclude that at any locality along the American coast the 

 shells will probably exhibit a greater variation in respect to 

 stature than at any locality along the British coast. 



A Second Test of Variability. 



It is quite possible that the American shells may be more 

 variable in respect to stature and still be less variable in 

 respect to other characters. 



In the previous section it was shown that in each American 

 locality the shells vary through a greater amplitude than in 



