No. 3-] VARIATION IX THE TEETH OF . \EKE1S. 139 



for the number of teeth in twenty-two species is lower than 

 the mean of total teeth in the one species which I have 

 described. The skewness of the curve instead of being nega- 

 tive is positive, although it is exceeding small. Had it been 

 negative, as I had thought it might be, it would have indicated 

 that in the species of the genus, as well as in the individuals 

 of the species, there had been a movement in the direction of a 

 smaller number of teeth, either from an excessive production 

 of individuals and species having few teeth, or from selective 

 annihilation of those having many teeth. The opposed positive 

 skewness is so small that it may mean little in regard to the 

 species, and particularly since the numbers are small and the 

 method of counting so doubtful no stress can be laid upon it. 



5. Abnormalities. 



Differences between the two jaws of the same animal in the 

 definite, indefinite, or total number of teeth were of common 

 occurrence. The accompanying drawings are intended to 

 show some irregularities of this kind. In Fig. i the right jaw 

 has four large definite teeth and five below which do not 

 emerge from the surrounding chitinous layer ; the left jaw has 

 only slight crenulations corresponding to five definite teeth, 

 although it has the same number of indefinite teeth as the 

 right side. Fig. 2 shows on the right jaw three definite 

 teeth, the edge above and distal to them having three very 

 slight elevations ; the opposite jaw ends in a long point with a 

 perfectly smooth edge and has only two large definite teeth 

 below. There is the same number of indefinite teeth on both 

 sides. Figs. 3 and 4 show variations of the same kind, the 

 numbers of both definite and indefinite teeth being different 

 for the two jaws. 



So far the drawings have been made from old animals in 

 which the jaws are hard, strong, and very dark in color. It is 

 probable from the appearance of the jaws that the difference 

 in the number of definite teeth is due largely to the wearing, 

 on one side or the other, of the distal teeth. Fig. 5 shows a 

 common irregularity of equal totals with slight differences in 



