Diversity in the Sci;tes of Chelonia. 41 



adjustment, there is evidence of post-natal partial division of scutes 

 by seams that lessen the abnormality in adjustment while they in- 

 crease the abnormality in niHnbev of scutes ? 



There are two classes of costal abnormality that might seem to be 

 exceptional and which, therefore, should be referred to here, (a) 

 When small supernumerary elements occur at the extreme anterior 

 or posterior ends of the costal series (Figs. 10, 21, 23, etc.), without 

 interfering with the symmetrical plan of the normal scutes we would 

 expect no change in the neural series — the adjustment of the series 

 is already practically perfect, (b) Another illustration of asym- 

 metry in number only is offered by the division of the 5th costal 

 (Fig. 27). Two scutes may occupy just the position of this shield, 

 without effect on the general plan of the series. On the other hand, 

 of course, asymmetry of costal s may occur w^ithout supernumerary 

 costal scutes. 



To sum up — 



In the normal symmetrical carapace neurals and costals have an 

 alternating relation, and the neurals dovetail between the costals. 

 In the imsymmetrical carapace, this relation prevails on each side. 

 It follows that one side of the neural series may be formed accord- 

 ing to a different plan than that of the other side. The two oppos- 

 ing plans are independent of each other, but never entirely so. The 

 scutes necessary to one plan extend more or less over into the other 

 side, sometimes even to the opposite costal series, but always in 

 reduced form. The two sides of the body show, at the same time, 

 a degree of mutual independence, with a degree of mutual de- 

 pendence. 



In consequence of these conditions of correlation it is rare to 

 find neural scutes of abnonnally great antero-posterior extent, and 

 there is noted a correspondence not between numbera of neurals and 

 of costals, but between the respective regions of abnonnality of the 

 two series. 



The value of these observations would be lessened if the sugges- 

 tion of the general applicability of the principle inferred would 

 conflict with previous observations, or with hypotheses which for 

 other reasons we must accept. Previous observations on land and 



