96 



BULLETIN 89, UNITED STATES NATIONAL MUSEUM. 



broken off and healed, while the third (No. 6646) appears to have suffered other 

 injury, as shown by the curved and thickened condition of the shaft. 



The bases of the posterior pair in an articulated tail, as shown by two indi- 

 viduals (pis. 15 and 16),' would be subadjacent to the seventh, eighth, and ninth 

 caudals, counting back from the tip of the tail, while the anterior pair wouJd be 

 above the twelfth, thirteenth, and fourteenth vertebrae. 



The relative difference in length between an anterior and posterior spine of 

 the same individual is shown in the table of measurements below : 



Measurements of spines. 





Greatest length. 



Anterior 

 spine. 



Posterior 

 spine. 





635 

 598 

 446 

 300 



mm. 

 522 

 577 

 384 

 280 











Arrangement op the Armor. 



The most striking fact concerning the arrangement of the armor and one con- 

 vincingly demonstrated by this specimen (No. 4934) is that the plates of opposite 

 sides are not arranged in pairs, but those of one side alternate with those of the 

 other (pi. 14).^ This feature has been the subject of considerable speculation and 

 discussion among vertebrate paleontologists and is one upon which directly opposite 

 views are held. 



Marsh, in 1891, made the first pictorial restoration^ of Stegosaurus (pi. 32, upper 

 figiure) and placed the series of plates in a single row along the median hue of the 

 neck, back, and tail, with four pairs of spines near the end of the tail. Lucas, in 1 901 , 

 published the next restoration,^ and was the first to show the plates arranged in 

 pairs (pi. 33, lower figure). Later, in a statuette prepared under his direction, the 

 plates were made to alternate along the back and the caudal spines were reduced 

 from fom- to two pairs. (See pi. 34, lower figure.) 



The latest conception, as exemplified by a recently mounted skeleton ^ in the 

 Peabody Museum of Yale University (pi. 36, loyer figure), reconstructed under the 

 direction of Prof. E. S. Lull, and a small model of the animal in the flesh has 

 been made after this mount (pi. 36, upper figui-e)." In this a return to the paired 



1 The positive evidence given by specimens in the National Museum as to the proper position of the spike-like spines near 

 the end of the tail in Stegosaurian dinosaurs would indicate that Nopcsa was in error in placing similar spines of Stegosaurus 

 priscus in an upright position on the scapular region of that animal. (See F. Baron Nopcsa, Geol. Mag., vol. 8, No. 4, 1911, p. 152. 

 fig. 9.) 



2 Lucas was the first to recognize this arrangement. In his Animals Before Man in North America, 1902, p. 171, he says 

 of the plates; "They were placed far enough apart to permit freedom of motion and appear to have been arranged alternately 

 and not in pairs." 



3 Amer. Joum. Sci., vol. 42, 1891, pp. 179-181, pi. 9. 

 * Animals of the Past, 1901 , fig. 24. 



Amer. Joum. Sci., vol. 30, 1910, pp. 361-377, figs. 2, 3, and 4. 

 5 Idem, pi. 11. 



