Fig. 69. — Diagrams illustrating the progressive reductioTi of the horny shields in varioxis 

 Clielouiaus. The shields, the fate of which it is desired to follow, are indicated by 

 distinctive shading. I. Hypothetical, primitive stage. Eight neural (including the 

 nuchal) and eight costal shields. Both neurals and costals lie in the same trans- 

 verse planes. II.-VII. Successive stages in the reduction and suppression of various 

 shields, observed iu specimens of Tludassochelys, the normal condition of which is 

 represented by VII. VIII. Six neurals and only four costals. The normal condi- 

 tion of Clidone. IX. The nuchal shield has become very small and the resulting 

 gap has been filled up by an enlargement of the first pair of marginals. This is the 

 normal condition of most Cryptodirous tortoises. X. The first marginals meet in 

 front and the nuchal is either suppressed (Xa), e.g. in several species of Testiulo, or 

 it is surrounded by the marginals (Xb), e.g. in Slernothaerus. (From Willey's 

 Zool. Results, 1899.) 



