14 ■ PEOCEEDINGS OF THE NATIONAL MUSEUM vol. 81 



Museum specimen there are reasons for believing they may be due 

 to senility. In the first place there is no distortion of the bones, 

 and while there is a small excess of extraneous bony matter extending 

 over the joints, it is in no way comparable to the lesion on sauropod 

 caudals described and illustrated by Moodie.^'^ 



Secondly, the ligaments connecting the spines have also ossified to 

 some extent in this solid section as well as in front of it, as is clearly 

 shown in Figure 3. Between the spines of Nos. 18 and 19 at their 

 bases a completely ossified ligamentary bar joins the two, as shown in 

 Figure 3. 



Why coossification of the vertebra should take place in this particu- 

 lar section of the tail is difficult of explanation. Hatcher explained it 

 as being the point where the tail first touched the ground and for that 

 reason was more susceptible to injury. With the dorsal elevation of 

 the tail, it no longer touches the ground in this region but posterior to 

 it, and thus this explanation no longer obtains. 



The tail as reconstructed in the present skeleton consists of 31 origi- 

 nal caudal vertebrae, 29 of which form a continuous series with the 

 sacrals. The other two vertebrae were found disarticulated, but not 

 far removed from the end of the above-mentioned series. These are 

 thought to represent the thirty-third and thirty-fifth, respectively. 

 The missing vertebrae, including the long whiplike extremity, have 

 been replaced by casts made from the composite tail of the Diplodocus 

 skeleton in the Carnegie Museum. 



In introducing these casts considerable disparity in size between 

 caudals of the same serial position in the two specimens was found, 

 and we were obliged to omit four vertebrae from the Pittsburgh series 

 on account of their larger size. In other words, the thirtieth as used 

 in the present skeleton is the thirty-fourth of the Carnegie Museum 

 specimen. This omission shortens the tail by 3 feet, as contrasted 

 with the Pittsburgh caudal series. 



A distal caudal centrum (Qu. No. 82) with some of the neural arch 

 found with the other scattered caudals at the end of the articulated 

 series could not be used because of its reduced length, being consider- 

 ably shorter than any of the casts of this section of the tail. Whether 

 this abbreviated caudal indicates a shorter tail in this individual, or 

 whether it does not pertain to the present specimen, is a question that 

 can not be determined until a complete articulated caudal series of 

 Diplodocus is discovered in the Dinosaur National Monument area. 

 At this time it seems best to omit it and complete the whiplike por- 

 tion with the casts of the Pittsburgh specimen which formed an 

 articulated - series, 



21 Amer. Journ. Sci., vol. 41, pp. 530-531, fig. 1, 1916. 



'^ Holland, W. J., Mem. Carnegie Mus., vol. 2, no. 6, p. 253, 1906. 



