94 



UNITED STATES NATIONAL MUSEUM. 



to them would give us 20 as the total number of these flat plates, whereas Marsh, 

 in his first restoration, has 12 in a single row (pi. 32, upper figure), Lucas 30, 24 and 

 22, respectively, in his restorations (pis. 33 and 34), and Lull 28 in the latest con- 

 ception (pi. 36). 



It is of interest to note that the plates of specimens Nos. 4714 and 4934 when 

 correlated on the basis of their relative positions as found in the field (compare pi. 2 

 and fig. 58) agree very closely in measm-ements and in general contours. This 

 correlation is shown below : 



Correlation of plates of the tail in two specimens. 





No. 4714. 





No. 4934. 



Greatest 

 longitu- 

 dinal 

 diameter. 



Greatest 

 vertical 

 diameter. 



Greatest 

 longitu- 

 dinal 

 diameter. 



Greatest 

 vertical 

 diameter. 



No. 95 



No. 10 



No. 75 



No. 76 



No. 177 



mm. 

 650 

 770 

 660 

 420 

 295 



mm. 

 580 

 70O 

 620 

 320 

 210 



P. 13.... 

 P. 14.... 

 P. 15.... 

 P. 16.... 

 P. 17.... 



mm. 

 695 

 785 

 630 

 410 

 290 



mm. 



760 

 627 

 335 

 225 



On the evidence of these two individuals it would appear that there were only two 

 of the sharp-edged overhanging distal plates (pi. 24, figs. 1 and 2) on the taU, whereas 

 , ^ the Yale mount shows six. In 



the first restoration by Marsh 

 none of this type were indi- 

 cated (pi. 32, upper figm-e). 

 Had such a plate been inserted 

 by him it would have necessi- 

 tated the removal of the two 

 anterior pau-s of spines in or- 

 der to make room for it, but 

 such a change would have 

 been in accord with the se- 

 quential order of these bones, 

 as shown by the evidence pre- 

 sented here. That the dimi- 

 nution in the size of the plates 

 from the largest to the small- 

 est was a regularly graduated 

 change and not an abrupt 

 transition, as seen in the Yale mount (pi. 36), is abundantly shown. 



I can not accept Lull's ^ statement, when, in discussing the distal plates of the 

 tail, he says "the base being embedded in the muscles between the neural spines and 



1 Verhandlungen des VUI. Internationalen Zoologen-Kongress zu Graz, 1912, p. 676. 



Fig. 59.— (1) Lakge dermal spine of Stegosaubus Ungulatus Marsh. 

 tV NAT. SIZE, a, Side view; 6, front view; c, section; d, inferior view 



op BASE. (2) SMALLER CAUDAL SPINE OF SAME, 

 OTHER LETTERS SAME AS IN NO. 1. AFTER MAKSH. 



POSTERIOR view; 



