MESOZOIC ERA 183 



2. The Stegosauria or plated dinosaurs possessed a double row of pro- 

 jecting plates down the back, and spikelike spines on the tail (Fig. 9.8). 

 We may imagine that the thrashing about of that tail must have aflforded 

 persuasive discouragement to carnivorous dinosaurs in search of prey. Al- 

 though Stegosaurus was quadripedal, bipedal ancestry had left its mark in 

 the disproportionately short forelegs (Fig. 9.8). The skull was inordi- 

 nately small, housing a brain about the size of a walnut. This in an ani- 



FIG. 9.8. Stegosaurus, the plated dinosaur; length about 20 feet. (From Lull, Organic 

 Evolufion, p. 484. Copyright 1945 by Richard S. Lull. Used by permission of The Mac- 

 millan Company, publishers.) 



mal bigger than an elephant! In the region of the hind legs was found an 

 enlargement of the spinal cord about twenty times as large as the brain. 

 It would seem that the brain must have served principally in connection 

 with the sense organs of the head and with the activities of the small, 

 weakly toothed mouth, leaving coordination of the remainder of the body 

 to the spinal cord. Many other dinosaurs also had sacral enlargements of 

 the cord greater than their brains. The Mesozoic era is not memorable for 

 intellectual activity. 



3. The Ankylosauria were heavily armored dinosaurs somewhat reminis- 

 cent of turtles or of armadillos in the completeness of their armor plate. 

 They have been called the "tanks" of the Mesozoic battlefield. 



4. The Ceratopsia or horned dinosaurs owe their name to possession of a 



