236 PROCEEDINGS OF THE NATIONAL MUSEUM. Vol. xxxvi. 



The centra of sacrals two and three are much compressed trans- 

 versely, as shown in fig, 17, S., and S.^^, the third being the smallest of 

 the five. Antero-posteriorly the lateral and ventral surfaces are 

 deeply concave. The inferior surface of sacral two is somewhat 

 pinched together inferiorly, Avhile sacral three is broader and flat- 

 tened in this aspect. 



The fourth sacral was firmly united to the fifth, although the 

 suture is plainly seen. Like those preceding, the sides are concave 

 antero-posteriorly, though the centrum as a whole is more robust 

 than either sacrals two or three. Its ventral surface is somewhat 

 flattened transversely, though concave longitudinally. Unfortunately 

 only the centrum of the fifth sacral is j^reserved in this specimen. It 

 differs but little from the fourth sacral, except that the rib or trans- 

 A'erse process is not borne intervertebrally, as are those preceding, but 

 is confined wholly to the anterior half of this centrum, Avliich has a 

 fragment of the articular end still attached, as will be seen by ref- 

 erence to fig. 17. The centi'uni is also more cylindroid in outliuo 

 and the floor of the neural canal is much constricted transversely, as 

 in the fourth of i'. {Iguanodon) prestwichii. The latter observa- 

 tion is also true of the fourth or last sacral (fifth sacral of Marsh) in 

 No. 1877<^/, Yale Museum, as is plainly shown in fig. 87, 8. A speci- 

 men referred to C. nanus, in the xlmerican Museum of Natural His- 

 tory, which has this region articulated, shows that the ribs or trans- 

 verse processes of this vertebra reached and gave support to the ilia. 



The fourth is the first sacral to show the peculiar peg-and-notch 

 articulation (see fig. 17), considered by Marsh as characteristic of 

 the sacral region of Caniptosant'us. He says:" 



The vertebrse of the sacrum, especially the posterior four, are joiued to each 

 iither by a peculiar peg-aud-notcli articulatiou. The floor of the ueural canal 

 of each vertebra is extended forward into a pointed process (somewhat like an 

 odontoid process), which fits into a corresponding cavity of the centrum in 

 front. This arrangement, while permitting some motion between the indi- 

 vidual vertebme, helps to hold them in place, thus compensating in a measure 

 for absence of ankylosis. 



This articulation may have been present between sacrals two and 

 three before ankylosis took place, but this can not now be determined. 

 It is, hoAvever, present between sacrals three and four, four and five, 

 sacral five and caudal one, and there are also faint indications of the 

 same method of articulation between caudals one and two, and two 

 and three (see p, fig. 18). 



In this connection it is of interest to know that an examination of 

 the type-specimens of Laosaurus consors and Dryosavrus altus shows 

 that both specimens exhibit the same peg-and-notch articulation of 



"Arner. Journ. Sci.. XLVII, 1S94, p. 246, 



