"ABNORMALITIES IN CHELONIA. 93 



4. Supernumerary Scutes on tlie Plastron. 



The plastron has, as a rule, reached a higher degree of fixity in 

 the matter of numbers and arrangement of scutes than has the 

 carapace, but that this portion of the chelonian armor has not al- 

 ways had so fixed a character may be seen in the high state of 

 variability of Aromochclys odorata, which is almost as marked as 

 that seen on the carapace of Thalassochelys. In Aromochclys the 

 number of plastron scutes varies from 14 to 9 and all intermediate 

 conditions are readily found. Fig. 5 2 shows the largest number of 

 scutes seen in the specimens of my collection. In this case there 

 is a well-developed extra pair of scutes between the abdominals and 

 femorals. Fig. 53 shows the commonest condition in which 

 there are the usual five pairs of plates and the gulars are par- 

 tially fused. Fig. 54 shows the extreme of reduction in which 

 the pectorals have been lost either through crowding or fusion, 

 and the gulars have fused into a single median element. 



As in the case of the carapace, we find in several species that 

 have attained a high degree of fixity in the plastron, marked 

 traces of supernumerary scutes. Fig. 49 shows the plastron of 

 a small specimen of Chelydra that has an extra pair of scutes be- 

 tween femorals and anals. Fig. 50 shows another specimen of 

 Chelydra with a vestigial scute on the right side between humeral 

 and abdominal. Fig. 51 shows a specimen of CJirysemys with a 

 well-marked supernumerary scute on the left side between ab- 

 dominal and femoral. In Figs. 49, 50, 51 and 52 we have super- 

 numerary scutes in four places out of a possible five. As yet I 

 have been unable to find supernumerary scutes between gulars 

 and numerals. 



Losses seem to have taken place in two ways : by fusion and 

 by crowding out. Some curious examples of the latter might be 

 mentioned. In Chelydra the abdominals have been forced to the 

 sides, but have been retained to bridge the gap between the small 

 plastron and the margin of the much larger carapace. In other 

 cases the pectorals have played a similar role. Van Lidth de 

 Jeude describes a specimen of Tcstudo ephippium (Gthr.), in 

 which the pectorals have been crowded to the two sides lika the 

 abdominals of Chelydra. Other specimens of the same species, 

 according to Rothschild, have the same abnormality to a greater 



