42 Eobert E. Coker. 



marsh turtles have been used above) ; those on sea-turtles will be 

 used in a later portion of this paper. 



The only hypothesis which has a bearing is that which regards 

 these abnormal scutes as atavisms. I believe that the general truth 

 of these observations would conflict with that explanation, unless 

 it could be supposed that the reversion was to a stage when neurals 

 displayed the asymmetrical condition observed. In any event, I 

 believe that the upholders of the hypothesis as applied to the scutes 

 in question should account for the following facts : 



1. That, generally speaking, more supernumerary neurals are 

 observed in specimens with supernumerary costals on one side than 

 in those with symmetrical supernumerary costals. 



2. That, while supernumerary scutes may occur in any one series 

 without additional scutes in any other series, thus indicating the 

 partial independence of each series, yet, when a Siupemumerary 

 costal occurs in such a position that the symvnetry of the costal plans 

 is seriously interfered with, the number of supernumerary neurals 

 will depend on the extent of the region of asymmetry. 



3. That we do not find asymmetry of costal plan without super- 

 numerary neurals. (See qualification above, p. 41). 



There is nothing in these observations to conflict with a supposi- 

 tion that the primary variation, the resulting adjustment of which 

 we see, may be an atavism. 



Referring again to the question of the identification of super- 

 numerary scutes — as costals or neurals, etc. — if the adjustment of 

 the whole carapace in accord with the laws of growth is the thing, 

 it is of little real significance whether a given abnormal scute be 

 termed "costal" or "neural," however convenient such a classifica- 

 tion may be for the practical purposes of description. 



We have vet to consider the neural and costal scutes in relation 

 to the bony plates beneath, but it is desirable to defer this until 

 after the study of the scutes of Tlialassoclielys, which must conclude 

 the present paper. 



