1911.] 



NATURAL SCIENCES OF PHILADELPHIA. 



141 



An examination of the measurements of the entire suite of Somerset 

 specimens shows that the distribution of dimensions follow this 

 grouping. From the plotting of the distribution of height and width 

 the relative numbers of the four types are seen to be as given below 



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Fig. 5. — Somerset. 



in the 229 examples studied. The separation into groups by the 

 width measurements is quite sharp, but, as is explained later, the 

 heights overlap somewhat: 



Range of Dimensions in the Four Types at Somerset. 



No. of 

 Type. examples. 



Somerset Extinct 12 



Somerset Gully 55 



Somerset Normal 127 



Somerset Hill-top 33 



The curves were plotted for the entire 229 specimens and no attempt 

 was made to sort them out into groups. But nevertheless the curves 

 show the types quite plainly. Thus the widths for the 33 hill-top 

 forms 13 are 42 mm. and 13 are 43 mm.; whereas the next millimeter 

 in diameter, 44 mm., is represented by 23 specimens, 45 mm. by 29 

 specimens, 46 mm. by 26, 47 mm. by 25, 48 mm. by 24; this range 



