868 THE AMERICAN NATURALIST. | [Vor. XXXIV. 
Ricut (Lower) VALVE be LEFT (UPPER) VALVE 
| | 
C. S.H. | Cutchogue | F. I. & O. I. | Cutchogue |F.I.& O.I. 
|- | 
n 1046 | 281 | 50 | 808 153 | 2t 
M 17 17 | 17 17 16 | 16 
A 17.353 + .o18 | 16.534 + .034 | 16.480 + .084 16.790 + .022 15.954 + .105 | 15. T. B, : 
o 0.876 + .013 0.852 + .024 0.877 + .060 | 0.916 +.015 | 0.881 + .075 | 0.97 
c 5.049 € .074 Bdg6 es 5.32; 36,36 5.457 + .092 5-52 3.47 | Mais p 
F 1.46 0.0476 | 
Type ET f Type IV or | 
| Type V1 | 
Skewness + 0.023 + -0000000058 | 
1 Fx M= .028; he tines Aa E .9711. 
v 
Conclusions. — From these numerical results we may draw 
the following conclusions : 
1. The right valve has on the average about half a groove 
more than the upper valve (more precisely .56+ more). This 
result is due to the circumstance that the series of ridges and 
grooves must end either in a ridge or in a groove. Of course 
it may end in a ridge at one end of the series and a groove at 
the other; but in the majority of cases there is a high degree 
of symmetry in the ends of the series. Now I find, in looking 
over the right and left shells without prejudice, that in the right 
shells the series tends strongly to end in a groove, so that the 
last or most lateral ridges of the series (looking at inner sur- 
face of shell) are very distinct. In the left valve, on the other 
hand, there is a more marked tendency for the series to end 
in a ridge, so that the last grooves are distinct. For in 
twenty-three right valves I found only three in which the series 
ended in ridges, whereas twenty ended in grooves; whilst in 
twenty-nine left valves in fourteen cases the series ended in 
ridges, and in fifteen cases in grooves.! Since the series of the 
right valve show this prevailing tendency to end in grooves, 
the excess of grooves on the right valves is fully accounted for. 
Doubtless if ridges had been counted instead of grooves, the 
right valve would have averaged one-half a ridge less than the 
left. 
l The tendency to end in a groove or a ridge is beautifully shown in P. oper- 
culatus Linn. 
