76 
A Study of Variation. 
scatter. (Fig. 4 A.) The remaining diagrams shew a 
distribution of the dots in two diagonally opposite quadrants. 
First the plotting of X 100 against X 100 is 
seen in Fig. 4 B. while the plotting of JggL x 100 against 
X 100 is seen in Fig. 5 the difference from the 
preceding one being that the diagonal of the scatter group 
is slightly steeper. 
Ribbing. — -The ribs of the pedicle valve are present in 
the very young specimens as a crimping along the posterior 
margin, but in the very large specimens they almost reach the 
umbone. The number of ribs in the pedicle valve of Pugnax 
pugnus (Martin) varies from o — 12 in the specimens observed. 
The number of ribs in the furrow is always one fewer than that 
of the ridge. In Pugnax cf. sulcatus (J. de C. Sow) the number 
of ribs in the pedicle valve ranges from o — 12, and the same 
fact of one more rib on the ridge than in the furrow is true in 
this case also. The ribs are developed first in the furrow and 
on the ridge, and become stronger on the sides with later 
growth. 
Conclusion. — In conclusion the following facts emerge, 
that : 
1. The group of six hundred specimens has been shown to 
consist of two species. This has been demonstrated by the 
frequency polygons. 
The Naturali • t 
