628 THE AMERICAN NATURALIST. | [Vor. XXXIV. 
EXPLANATION OF PLATE I. 
L. I. Polygons of variation of numbers of spines; empirical polygons are 
given by continuous, theoretical ones by dotted lines. 
A, ventral spines of P. varians. 
B, ventral spines of P. vulgaris. 
C, dorsal spines of P. varians. 
D, dorsal spines of P. vulgaris. 
e, index of variability of polygon above. 
ya, ym, yo, ordinates of theoretical curves to A, M, O, of abscissa; compare 
explanation of letters used in table, p. 
