328 CHELONIA 



CHAP. 



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 ! 



Before leaving the description of the shell, it is worth while 

 to draw attention to the enormous correlative changes in other 

 organs produced by this case. Nearly the whole organism has been 

 altered. The hard, firm carapace has partly rendered the supporting 

 functions of the vertebral column unnecessary or impossible. In 

 many tortoises, especially in the large land-tortoises, the vertebrae 

 and the capitular portions of the ribs are reduced to mere bony out- 

 lines ; the reduction to thin paper-like bony lamellae proceeds with 

 age. The iliac bones find a better support in the costal plates ; 

 the contact with the sacral ribs is given up, and these ribs fuse 

 partly with the costal plates, or they are absorbed. The whole 

 mass of muscles of the trunk is completely lost in the region of 

 the shell, but traces of them exist in young specimens. Neck, 

 limbs, and tail can in most cases be withdrawn and hidden in 

 the shell When this is not possible it is due to secondary 

 changes. The neck is withdrawn either by being tucked away 



