326 CHELONIA 



The carapace of most Chelonians is covered with five neural, 

 four pairs of costal and twelve pairs of marginal shields, the last of 

 which often forms an unpaired pygal. In front of the first neural 

 lies the nuchal shield, very variable in size, often absent. The 

 Chelydridae, Dermatemydidae, Platysternidae, and Cinosternidae 

 possess moreover several inframarginals, intercalated on the bridge 

 between the marginal and some of the plastral shields. In many of 

 the other families these inframarginals are restricted to the anterior 

 and posterior corners of the bridge, as the so-called axillaries and 

 inguinals, mostly small and variable. Lastly, Macroclemmys has 

 several small supramarginals. 



There are consequently eleven longitudinal rows of shields 

 in all ; by elimination of the supra- and infra-marginals they 

 are reduced to seven rows. It is absolutely certain that the 

 number of transverse rows also was originally much greater than 

 it is now. The mode of reduction of the number of the neural 

 and costal shields has been studied in Thalassochelys carctta (cf. 

 p. 388.) The accompanying illustration (Fig. 69) shows some of 

 the main stages actually observed in the reduction of these shields. 

 The chief point is that certain shields are squeezed out, or sup- 

 pressed by their enlarging neighbours. The ultimate result is 

 the formation of fewer, but larger shields. 



Each shield grows individually as follows. Every year, or 

 rather during every periodically recurring period of growth, the 

 area of the Malpighian layer belonging to each shield increases 

 peripherally in size, and at the same time produces a new layer 

 of horn. The original little shield, with which the tortoise is born, 

 remains for years, often throughout life, as the so-called " areola ; " 

 it increases in thickness owing to the new layer of horn added 

 from below, and peripherally the increase in size is indicated by 

 the overlapping concentric rings. Each ring represents a year's 

 growth, at least in tortoises which live in temperate zones, where 

 hibernation means a complete suspension of growth. It is not 

 known if the same apj)lies to tropical species, which grow either 

 throughout the year, or which undergo one or more periods of 

 rest. The areola does not remain central ; the growth is uneven. 

 With age the oldest layers of the areola are frequently rubbed off, 

 and the areola then appears enlarged. 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, 



