{327}[Illustration: 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 transverse planes. II.-VII. Successive stages in the reduction and 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 condition 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 (X_a_[X^a in diag]), _e.g._ in several species of _Testudo_, or it is surrounded by the marginals (X_b_), _e.g._ in _Sternothaerus_. (From Willey's _Zool. Results_, 1899.)] {328}For the first dozen years or so the annual rings can be easily followed, but when the creature approaches maturity each shield adds very little to its growth, and the rings become very fine, crowded and irregular. Only by careful counting and comparison of the rings on the costals, marginals, and plastrals, can a reliable average be arrived at. In some tortoises, e.g. _Chrysemys_, the whole outer layer of the shields peels off periodically; only a thin smooth layer like mica or tracing-paper remains, of course without any indication of rings. The pigment is formed in the Malpighian layer, but it frequently diffuses into the horny shields themselves, notably in _Chelone imbricata_, which yields the beautiful "tortoise-shell." The colour of the pigment is either black, yellow, or red, with resulting combinations. The green colour, often so beautiful in baby-specimens of _Chrysemys_, is optical, produced, according to Agassiz, by a network of black pigment, spread over a layer of yellow oil. Horny scales, sometimes forming spines, and covering a nodule of dermal ossification, are also common on other parts of the skin, especially on the limbs of land-tortoises, and also on the tail of _Chelydra_. Sometimes the end of the tail is protected by a claw-like nail, for instance in _Pyxis_. In some of the gigantic land-tortoises, and in _Chelone mydas_, this nail assumes large dimensions, and several of the terminal caudal vertebrae are fused together into a regular urostyle. In some subfossil specimens of Mauritian tortoises, these ankylosed complexes are 12 cm. long and more than 5 cm. broad!