94 H. H. NEWMAN. 



or less degree. The Catalogue of Shield Reptiles in the British 

 Museum shows a specimen of Mononria fnsca (Tab. III.), in 

 which the pectorals are crowded to the margin of the plastron 

 and have become small and triangular. The same volume shows 

 a specimen of Sternotherus Derbianns (Tab. XXII.), in which the 

 pectorals seem to have a tendency to be suppressed or crowded 

 to one side. 



On the whole it seems evident that an orderly suppression of 

 alternate scutes has taken place in the plastron as well as in the 

 carapace. 



5. Correlated Abnormalities in tJie Scutes and Bony Plates. 



The next question that comes up for discussion is whether or 

 not there is any correlation between scutes and bony plates. It 

 has long been noticed by morphologists that there is a certain 

 definiteness about the relative positions and sizes of scutes and 

 plates. This may be described in brief as a definite overlapping of 

 bony sutures by scutes. In the marginal series (see Fig. i) this 

 is seen in its simplest form every bony suture being covered 

 by a scute. In the neural and costal series one scute as a rule 

 covers one whole plate and half of two adjoining plates. This 

 arrangement is modified in the anterior and posterior regions. In 

 the former the nuchal plate is partially overlapped by six scutes, 

 viz. : nuchal, first pair of marginals, first pair of costals (normally 

 involving only small corners of the plate), and the first neural. 

 The first costal scutes cover the first and half of the second cos- 

 tal plates as well as the inner edges of first, second, third and 

 fourth marginal plates. The last neural covers normally parts 

 of eight plates, viz. : the two procaudals and the anterior margin 

 of the pygal, about half of the eighth neural and eighth pair of 

 costals, and the anterior margins of the eleventh marginals. 

 Only in the middle portions of the carapace is any definiteness 

 of arrangement seen, yet there is a marked fixity of relations even 

 in the most specialized regions. Gadow bases his reduction series 

 upon an arbitrary connection between these structures, according 

 to which there was originally a scute for each vertebra and rib. 

 He gives no reason for assuming a vital connection between these 

 structures, but simply implies one. In an earlier portion of the 



