676 



Dritte Sitzung der sechsten Sektion. 



neural spines on the anterior third of the tail. Here the character 

 of the neural spines abruptly changes, the broadened summits 

 being lost and the whole spine very much reduced in height. 

 This point which marks the beginning of the aggressive, flexible 

 portion of the tail marks also an abrupt change in the character 

 of the plates which are now sharp-edged and pointed and bent 

 backward, the base being embedded in the muscles between the 

 neural spines and the vertebral centra. The sharp-edged plates, 

 of which there are three pairs, are followed by four pairs of long 

 spines similarly attached and pointing outward and backward at 

 a decided angle. The longest of these is 63,5 cm. (= 25 inches) 

 and its present weight about 6y 2 kilos ( = 14% pounds). 



The character of the surface of the thin expansion of the 

 plates, which doubtless represents the hypertrophied keel such 

 as one sees in the crocodile and in the other armored dinosaurs, 

 and the surface of the spines as well, all of which bore deep blood- 

 vessel impressions, implies a close fitting horny integument which 

 must have extended some little distance beyond the limits of 

 the bone. The character of the base in both plates and spines, 

 while indicating no articular connection with the underlying endo- 

 skeleton, nevertheless gives evidence of a firm cushion of carti- 

 lage, of special erecting muscles, and of great thickness of the skin. 



There are isolated, highly rugose ossicles, doubtless of dermal 

 origin, associated with the skeleton which cannot be placed with 

 any degree of assurance, but which may have been scattered 

 somewhat at random over the skin or concentrated at especially 

 vulnerable portions of the body. 



IV. The Nervous System. 



One of the most remarkable features of this unique reptile 

 lies in the form and proportions of the central nervous System 

 as shown by internal casts and measurements of certain regions 

 of the neural canal. 



The brain is remarkable for its extremely small size, the 

 entire cranial cavity having a length of 10,5 cm. and a width 

 of 3,0 cm., displacing but 56 c. cm. of water and with an estimated 

 total weight of about 70 gr. — this for an animal the total weight 

 of which must have exceeded that of the greatest of living ele- 

 phants whose brain averages 3,6 kilos or over fifty times the 

 weight of that of Stegosaurus. 



In comparing the relative potential intelligence of the two, 

 one has also to bear in mind the great preponderance of the cere- 

 brum in the elephantine brain over the other parts while in Stego- 

 saurus the cerebrum constitutes hardly more than a third of 

 the entire brain weight. The stegosaur brain has a very large 

 olfactory portion, small cerebellum, large medulla, and a hypo- 



