Fig. 69. — Diagrams illustrating the progressive reduction of the horny shields in various 

 Chelonians. 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. IT. -VII. Successive stages in the reduction aud suppression of various 

 shields, observed in specimens of Thalassochelys, the normal condition of which is 

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

 tion of Chelone. 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 Testthdo^ or 

 it is surrounded by the marginals (X&), e.g. in Sternothaerus. (From Willey's 

 Zool. F^sults, 1899.) 



