Diversity in the Scutes of Chelonia. 67 



Salient features of the observations on Thalassochelys : 



(1) The very high proportion of abnormality in these turtles, 

 which developed chiefly under abnormal conditions. 



(2) The general unsymmetrical aspect of the abnormalities, the 

 marginal, Mx, being an exception, since it occurs on both sides more 

 frequently than on one side alone. 



(3) The occurrence of small "supramarginals" — not previously 

 recorded as an abnormality ; these are always at the meeting point 

 of three other scutes. 



(4) The development of an embryo with only 3 scutes in each 

 dorsal series (Fig. G6), and another with 3 scutes forming the entire 

 carapace. 



(5) The appearance of a number of "monstrosities" — "cyclopean" 

 embryos, etc. ; and the association of a high proportion of abnormality 

 of scutes with the occurrence of these monstrosities. 



Supernumerary scutes occur in the neural and costal series with 

 much irregularity and without symmetry. The only symmetrical 

 recurring scute in either series is the rectangular scute posterior to 

 N4. At least two abnormalities in the costal series show a regularity 

 in their recurrence: these are — the absence of the small Cl, and the 

 presence of two scutes in the place of C5 ; but neither of these abnor- 

 malities appeared on both sides and symmetrically. 



Part III. Significance of the Abnormalities. 



Introduction. 



The specimens of Thalassochelys observed display an even greater 

 degree of diversity in number and arrangement of scutes than did 

 those of Malaclemmys. While Newmann found that one-tenth (48) 

 of 476 specimens of Graptemys had supernumerary scutes in the 

 carapace, and only about one-twentieth of 188 specimens of Chrysem- 

 mys, about one-fifth of the first 243 specimens of Malaclemmys 

 observed had more or less than the typical number of scutes. But 



are intercalated, generally in association with intercalated costals, sometimes 

 independently, but not with marked asymmetry. In fact, the only abnormali- 

 ties in my specimens that resemble those of his are the presenece of 2 scutes 

 in place of C5, and the interposition of a small neural posterior to N5. 



