ADAPTATIONS 127 



functions of the vertebral column unnecessary or impossible. 

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

 vertebrae and the capitular portions [heads] of the ribs are re- 

 duced to mere bony outlines ; the reduction to these paper-like 

 bony lamellae proceeds with age. The iliac [upper pelvic] 

 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 can in most cases be withdrawn and hidden in the shell.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 is withdrawn either by being tucked away sideways (Pleurodira), 

 or by being bent in an S-shaped curve in a vertical plane 

 (Cryptodira). In a left-sided profile view of the animal, the 

 head represents the tail of the S. The neck is withdrawn by 

 long muscles, which are inserted into the ventral side of the 

 middle of the neck, and extend in the shape of vertical ribbons 

 far back into the shell, arising from the centra [bodies] of some 

 of the middle or even more posterior thoracic [trunk] vertebrae." 



It should be added that certain giant land-tortoises inhabiting 

 the Galapagos Islands (where there are no large animals to 

 attack them) have undergone a kind of retrograde development 

 in the matter of dermal armour, the bony plates of the cara- 

 pace having become so thin that they can be pierced with a 

 penknife. This is an instance of the tendency in nature to dis- 

 pense with an organ or structure when it is no longer of use. 



A few of the Triassic anomodont reptiles, such as Elginia 

 and Geikia have developed horn-like processes on the skull 

 comparable to those of the horned dinosaurs, and perhaps also 

 sheathed in horn during life. Whether these reptiles had a 

 dermal armour does not seem to have been ascertained. 



