32 . Robert E. Coker. 



regarding the abundance of such forms.) A young specimen of 

 Chelydra in the zoological museum of the Johns Hopkins University 

 has its keel marked with a distinct sagittal furrow which extends 

 from the anterior margin of JSTI to the posterior margin of 'N5, and 

 is continuous through all of the keel spines except that of N5 ; such 

 a furrow is seen less distinctly in the shells of older turtles. 



Ill the light of these median furrows and incomplete and complete 

 seams, and of the parallel keel prominences of Xo. 151, there would 

 seem no inherent improbability of duplication of the median keel or 

 of neural scutes, nor any necessity for explaining away appearances 

 of duplication. To jump to the other conclusion, that the asym- 

 metrical neurals are primarily paired scutes which are crowded out 

 of a symmetrical plan, would be equally unwarranted. ''Crowding" 

 may be, at best, but a descriptive term referring only to the appear- 

 ance presented, rather than to any organic phenomenon.'^ 



May it not be that we have to do here neither with scutes in 

 sequence nor with paired scutes, but with a real asymmetry of scute 

 plan ? Symmetry is a normal feature of the carapace, but the cases 

 in question are admissably abnormal, and, on the face of it, the 

 scutes are asymmetrically disposed. The question is thus : Is the 

 visible asymmetry secondary and due to the crowding out of line 

 of elements that are primarily symmetrical, or is there in such 

 cases a real primary, though abnormal, asymmetry, which is perhaps 

 correlated with some other asymmetry ? The almost invariable 

 association of asymmetry in number or arrangement of the costal 

 scutes is strongly suggestive of the latter conclusion. Granting some 

 primary abnormality arising either as a germinal variation or in con- 

 sequence of environmental conditions, it may be that the conditions 

 of gro'wth cause the development of the neural scutes neither in linear 

 sequence nor in pairs, but in essentially such an asymmetrical plan 

 as is presented by the cases in question. 



these nine terrapin fifteen neural scutes, besides nuchals, sliow some degree 

 of division. Hay states that the longitvidinal division of scutes was "so com- 

 mon that it was really difficult to pick out a full-grown specimen which did 

 not show it in some degree." 



"Extra marginals may be small or large but do not seem to be "crowded 

 out," and the same may be said of extra costals. 



