ADAPTATIONS 123 



creasing in size from the nape of the neck to the loins, where 

 they are about a yard high and as much in basal length. To- 

 wards the tail they again begin to diminish in bulk, and on the 

 terminal part of that appendage they are replaced by four or 

 five pairs of long and stout spines, which may be compared to 

 marline-spikes. Structurally these vertical plates appear to be 

 an ultra development of the longitudinal low keels on the 

 plates of crocodiles. 



Stranger still is the armature of the horned dinosaurs of the 

 Cretaceous of North America, as represented by Triceratops 

 flabellatus and its allies, which mark the culminating point of 

 dinosaurian development in the way of armature. Triceratops 

 takes its title from the presence of three bony horn-cores on 

 the skull, of which two form a pair above the eyes, while the 

 third is situated on the nose, after the manner of the horn of 

 the Indian rhinoceros. The most extraordinary feature in the 

 skeleton of these reptiles is, however, the huge flange-like 

 shield projecting backwards from the occipital region of the 

 skull, and completely covering the anterior vertebra; of the 

 neck. In life, it seems most probable that the bony horn -cores 

 were ensheathed with horn, in which case this armature would 

 be precisely comparable to the horns of oxen and antelopes 

 among ruminant mammals. In some of these monsters the 

 length of the skull, inclusive of the neck-shield, was little short 

 of seven feet. Small dermal bony plates extended some 

 distance behind the posterior margin of the neck-shield, but 

 the body of Triceratops was not protected by armour. On the 

 other hand, certain allied types, such as Nodosaurus, appear to 

 have been furnished with a nearly complete panoply. 



The most typical of all armoured reptiles are the members 

 of the order Chelonia, that is to say the turtles and tortoises, in 

 which a " shell " of bone is developed from dermal elements, 

 and more or less completely encloses the body. To describe 

 in detail the structure of this shell would be out of place, and it 

 must suffice to say, firstly, that it consists of an upper half, or 

 carapace, and a lower moiety, or plastron. In some cases, as 

 in ordinary land-tortoises, the carapace is joined to the plastron 

 by a bony bridge, so that the whole structure forms a solid box 

 into which the head, limbs, and tail can be withdrawn ; the horny 

 beak and the horny shields on the limbs effectually guarding 



