82 H. H. NEWMAN. 



Starting with this ideal condition as stage I., he finds the 

 nearest approach to it in specimen I, that has 8 left costals of 

 which 2 are vestigial, 8 right costals of which I is vestigial, and 

 8 neurals. The greatest reduction is that seen in specimen 26, 

 which has 5 left costals of which I is vestigial, 4 right costals, 

 and 7 neurals of which I is vestigial. This latter specimen is 

 reduced below the normal for the species, which is arbitrarily 

 said to possess 6 neurals and 5 pairs of costals. This condition 

 is said to be the goal toward which every young Thalassochelys 

 carctta is striving. 



The following stages are mapped out in diagram, following 

 Gadow, to show the sequence in scute reduction in the chelonian 

 carapace : 



Stage I. - - Hypothetical, eight neurals and eight pairs of 

 costals. Neurals and costals lie in the same transverse plane 

 and coincide with neurals and costal plates. 



Stage II. - - Eight neurals and eight pairs of costals, the latter 

 fitting with their inner angles dovetailed between two successive 

 neurals. Rearrangement probably brought about by the partial 

 reduction of one pair of costal scutes. This reduced pair is 

 probably the second. 



Stage III. - - Eight neurals and seven pairs of costals, the 

 original second costals suppressed, original third becoming 

 second, etc. 



Stage TV. Seven neurals and seven pairs of costals, but fifth 

 neural and fourth pair of costals (original fifth), in a state of 

 reduction. 



Stage V. Six neurals and six pairs of costals, owing to 

 complete suppression of fifth neural and fourth (original fifth) 

 pair of costals. 



Stage VI. --Six neurals and five pairs of costals, brought 

 about by fusion of last two pairs of costals into one or, perhaps, 

 by suppression of one pair. This is the normal condition in 

 Thalassochelys. 



Stage VII. Six neurals and four pairs of costals. Normal 

 condition in the majority of tortoises to-day, brought about by 

 suppression of first pair of costals. 



Stage VIII. Six neurals and four pairs of costals, first neural 

 (nuchal) greatly reduced. 



