H. H. NEWMAN. 



tendency, however, would bring about the suppression of anterior 

 scutes, as a whole, before posterior scutes. 



Evidences are not wanting that scutes may be suppressed and 

 the method of suppression seems clear. In a specimen of Cycle- 

 mys dentata, listed as no. 58 and figured on Plate III., Fig. 42, 

 the paired sixth costals are being encroached upon by the seventh 

 costals. The anterior growing margins of the latter have pushed 

 in under the posterior edges of the former in such a way as to 

 severely cut into their growth centers. The dotted line shows 

 the amount of encroachment. Several specimens of Graptemys 

 show the same phenomena, and the scutes encroached upon are 

 always the supernumerary ones. This may be looked upon as a 

 recurrence of an ancestral condition and we may infer that the 

 loss of certain scutes has been brought about through the 

 encroachment, more and more severe with succeeding genera- 

 tions, of more vigorous upon less vigorous scutes, resulting in 

 the final complete suppression of the latter. We must also 

 suppose that the rudiments of the lost scutes lie dormant in the 

 embryonic tissues and occasionally for some reason reappear 

 more or less completely. Those that have been suppressed for 

 the longest time would naturally reappear least often and vice 

 versa. On this basis, then, we may safely say that the order of 

 loss is orthogenetic if by this we simply mean onward develop- 

 ment. 



Applying the same methods to Gadow's figures I find a very 

 general agreement, although 1 am unable to agree with the 

 author's interpretations. The vestigial scutes that occur in 

 Gadow's figures are : neurals 2, 8 and 10; costals I, 2, 4, 6, 8 

 and 10. No. 2 costal was not found in my specimens, but is so 

 clearly seen in Gadow's Fig. I that I have introduced it into my 

 system. It is possible that No. 2 costal was the most ancient 

 loss and hardly likely to recur in specialized types such as 

 Graptemys and C/iiysernys, since it occurred only once in Gadow's 

 specimens of TJialassoclielys. It will be seen that Gadow finds 

 no vestiges of neurals 4 and 6. An examination of his figures 

 will show that TJialassochclys has attained a high degree of fixity 

 in the anterior portions of the mid-neural series, while all other 

 regions are still in a decidedly variable condition. Hence we are 



