214 



ORTHOGENETIC VARIATION IN THE SHELLS OF CHELONIA. 



while in almost every one of those specimens with six neurals and reduced 4th costals 

 no corresponding reduction in the size of the neurals is visible. The only exception 



is specimen No. 24 (a 4 inch shell) which, as explained on p. 210, is abnormal in 

 several respects. The obvious way of accounting for this want of correspondence 

 between reduced 4th costals and an apparent reduction of a neural is that in these 

 specimens the original 5th neural has already been completely suppressed, anyhow 

 that it is wanting. Reduction of the last but one neural reduces a specimen from 

 eight to seven neurals, and reduction of the original 5th reduces such a shell to one 

 with six neurals only. In a similar way the number of eight or seven costals is 

 reduced to six by suppression of the 4th pair of costals. 



Let us now examine those Turtles which possess the normal number of six neurals 

 but still with six pairs of costals instead of the final number of five costals. There 

 are not less than 15 such specimens, all new-born. In some of them, for instance, 



