ABNORMALITIES IN CHELON1A. 



8 9 



The fact that supernumerary scutes have been found between all 

 of the normal scutes as well as at both ends of the costal series 

 must have some significance. If one assumes that these super- 

 numerary scutes represent the atavistic recurrence of scutes that 

 have been lost in the course of phylogeny, it is possible that the 

 following tabulation will throw some light on the sequence of 

 loss. 



It will be seen that the most frequent recurrences are at the 

 posterior end of the carapace, and that, with the exception of the 

 first costal, the frequency of recurrence diminishes as we proceed 

 anteriorly. What significance attaches to this fact ? It seems 

 quite probable that the most frequent recurrences represent the 

 most recent losses and the rarest recurrences the most ancient 

 losses. This rule held good for the suppression of rows of 

 scutes and should apply here also. 



On this basis then we can at least say that the succession of 

 suppression was in general antero-posterior, that the earliest 

 losses occurred at the anterior end of the carapace and the most 

 recent losses at the posterior end. One might go further and say 

 that in the neural series the order of suppression was probably 

 2, 4, 6, 8 and 10. The antero-posterior order of loss is not so 

 clear in the case of the costals, as no. I costal recurs more fre- 

 quently than any other except no. 10. This means a modifica- 

 tion of the regular mode of progression. In the costal series it 

 is probable that the antero-posterior succession of losses was 

 interfered with by the rounding-in of the marginals both ante- 

 riorly and posteriorly. This rounding-in would necessarily 

 begin about medially and proceed in two directions, hence the 

 second supernumerary scute would be put under pressure before 

 the first and the eighth before the tenth. The antero-posterior 



