252 BUM PUS. [VOL. I. 



variable, and, if it be objected that by chance we have selected 

 British shells from localities where they are least variable and 

 American shells from localities where they chance to be most 

 variable, the objection is met, if the same results follow upon 

 the examination of a more representative collection of shells 

 from either locality. Let us continue, and see if it is possible 

 to find a single American locality where the variation is 

 restricted even to that of the most variable British series. 



Chart VII. - - One thousand shells from Nahant, a rocky 

 promontory lying about ten miles south of Beverly and bathed 

 by the cold waters of the Atlantic, yield another characteristic 

 American curve, of low altitude and of broad base. The index 

 of the amplitude of variation is 19 (102 --83= 19), again in 

 excess of that of the British shells. The right limb is less pre- 

 cipitous than the left, but not so obviously as in previous cases. 



Chart VIII. - -This chart represents the distribution of 1000 

 shells from Plymouth, Mass. While the Littorinas of Nahant 

 were subject to the continual beating of the waves, those col- 

 lected at Plymouth were from the wharves of a sandy harbor, 

 far removed from the boisterous sea. The curve is peculiar in 

 that 213 shells, occurring at the ordinate of 90/0, have caused 

 an abrupt break in its contour, and formed a prominent spire 

 to what would otherwise be a characteristic American curve. 

 The ascent of the left limb is again considerably more abrupt 

 than the descent of the right, and the amplitude of variation 

 (17) is clearly American. 



Chart IX. - - In the warmer waters of the southern shores 

 of New England, at the rocky headland of Seaconnet, Lit- 

 torina actually swarms. The curve of distribution, though 

 drawn far to the left and indicating tall, elongated shells, has, 

 nevertheless, the characteristic American contour, low altitude, 

 flaring base, and the ascent on the left more abrupt than the 

 descent on the right. The amplitude of variation is as great 

 as it was for the shells from Beverly; viz., 20 (100 80= 20). 



Chart X. --Six miles to the west of Seaconnet, and largely 

 of the same geological formation, are the equally rocky prom- 

 ontories of Newport. The temperature and salinity of the 

 water, the oceanic currents, the force of the waves, the facies 



