"ABNORMALITIES" IN CHELONIA. 83 



Stage IX. Five neurals and four pairs of costals, first neural 

 (nuchal) suppressed as seen in pleuroderous tortoises. 



Beyond this last stage chelonians have not ventured yet, at 

 least normally. 



The order of loss in scutes is according to Gadow : (i) No. 2 

 costals, (2) no. 7 neural, (3) no. 5 neural and no. 4 (original 

 no. 5) costals, (4) no. 7 or 8 costals (by fusion or suppression), 

 (5) no. i costals, (6) no. i neural. 



Gadow's paper, while most suggestive, must be criticised in 

 several particulars, but before proceeding to the criticism it will 

 be necessary for me to produce the data that to a large extent 

 form the basis of the criticism. The data are derived from a col- 

 lection of a large number of abnormal specimens, principally of 

 two species, Graptemys geographica and Chrysemys marginata. 

 Gadow worked on a species that is normally abnormal if such 

 an expression be permissible. He selected the commonest con- 

 dition and arbitrarily called it normal. As a matter of fact, there 

 is no normal or fixed condition. The species TJialassochelys 

 cwetta is evidently in a highly variable state as to scute number 

 and arrangement, and no stability has as yet been attained. The 

 species I have studied have, on the contrary, reached an advanced 

 state of stability. Yet a sufficiently large number of abnormali- 

 ties occur to give one nearly as many examples as Gadow had. 

 Out of 476 specimens of Graptemys, varying from embryos to 

 adults and taken at random, there occurred 48 specimens with 

 supernumerary carapace scutes, while iSS CJirysemys yielded 8 

 such abnormal specimens. Four other species belonging to 

 widely diverse groups yielded one abnormality apiece. It seems 

 probable that abnormalities of exactly the kind that I have 

 found so plentifully in the case of Graptemys and Cliryscmys are 

 to be found in any species if enough specimens be examined. 



In order to economize space in the tabulation of these abnor- 

 malities brevity in the nomenclature of these vestigial scutes must 

 be attained by numbering them. Combining Gadow's figures 

 with my own results, I have good reason to believe that vestigial 

 scutes occur between every two surviving normal scutes and that 

 the first, second and last costals are also found in a vestigial con- 

 dition. On this basis, then, there were eleven neurals and ten 



