68 Kobert E. Coker. 



in 208 specimens of Thalassochelys, almost one-tliird were abnormal. 

 The diversity in Thalassochelys is even greater than this proportion 

 indicates, for, in estimating that proportion of abnormality, both 

 twelve and thirteen marginals were considered as normal. The 

 variable M2 is absent almost exactly as often as it is present and in 

 one-fourth of the total number of specimens it is absent on one side 

 while present on the other. It seems remarkable that there should be 

 such diversity in forms that have been so little modified throii^'h 

 several geological ages. Tlialassoclielys is at least as old as Eocene 

 times, and turtles of the Upper Jura have essentially the same num- 

 ber and arrangement of scutes as have modern turtles. Erom one 

 point of view the arrangement of scutes is exceptionally plastic or 

 variable, while from another, the paleontological point of view, it is 

 exceptionally persistent and fixed. 



It would be interesting to know the explanation of the "abnormali- 

 ties," such a large proportion of which show asymmetry, but the 

 data that we have in hand are, comparatively, very scant. It is 

 advisable, however, to consider the explanation advanced hitherto 

 (atavism) and other possible explanations in the light of the data in 

 hand. 



External Conditions. 



We have not sufiicient evidence as to the influence of the environ- 

 mental conditions of the eggs during development in the production 

 of abnormalities of scutes. A large proportion of abnonnalities 

 was found in each of the nests transplanted into the ground at the 

 laboratory, all of which were under more or less unfavorable condi- 

 tions. The nest which showed by far the largest proportion of 

 abnormal specimens was one in which very great pressure from the 

 distension of the eggs was noted. The smallest per cent of abnor- 

 mality yielded by any nest in the ground was found in the turtles 

 from eggs left undisturbed to hatch under natural conditions; but 

 the most normal lot of turtles of all was that obtained from eggs 

 which developed in the "incubator," with the element of pressure 

 eliminated. The series is inadequate for positive conclusions, how- 

 ever, as the number of embryos is too small, but it suggests the 

 importance of further experiments. 



