Diversity in the Scutes of Chelonia. 45 



10. Except in misshapen specimens, supernumerary marginals 

 seem to occur only anterior or posterior to the region of the dorsal 

 ribs. They are more common in the posterior region. A reduced 

 number of marginals results from the lack of one or a pair of the 

 posterior scutes. The alternating relation of scutes and plates is 

 well preserved in several specimens. 



11. Supernumerary costals may occur at the extreme anterior or 

 posterior ends of the neural series, without disturbing the general 

 plan of the series, or they may occur in such a way that the plan 

 of the series is altered. The two series of costals may be asym- 

 metrical even when the number of costal scutes is normal. 



12. Neural scutes may be partially or completely divided by a 

 median seam. This abnormality, though rare in the terrapins of 

 ISTorth Carolina and Maryland, is very noticeable in those of Texas 

 (Hay). 



13. Asymmetrical scutes in the neural series are of not uncom- 

 nion occurrence. There is observed an adaptation between asym- 

 metrical neurals and asymmetrical costals that strongly suggests that 

 the explanation of these scutes is to be sought in the adjustment of 

 scutes consequent on some more primary asymmetry. I do not 

 regard them as belonging in linear sequence and crowded out of 

 position. 



14. I believe that the asymmetrical neural scutes cannot be 

 regarded, individually, as atavisms. Their adaptation to an unsym- 

 metrical carapace seems too clear to permit of explaining them by 

 reversion to any earlier symmetrical plan of scutes. 



15. In the asymmetrical scutes we may find an illustration of 

 the fact that we may best interpret a single variation not by regard- 

 ing it as an isolated unit, but by viewing it in its relations to the 

 associated structures with which it helps to form a more or less 

 well co-ordinated whole. 



