208 



PROF. H. Gr. SEELEY ON A SACRUM OE A 



long concave lateral depressions, in which are impressed ovate 

 apertures of a pneumatic character (fig. 3, pn). 



The vertebral centrums are nearly equal in length, but become 

 slightly shorter posteriorly, and the sixth is conspicuously short 

 (fig, 2). The first has a length of 17 millim., the second of 16 millim., 

 the third, fourth, and fifth are slightly shorter, and the sixth measures 

 13 millim. 



The general aspect of the underside of the centrum is flattened, 

 with the flattening augmenting posteriorly. The first centrum is 

 decidedly convex from side to side and concave in length, but the 

 longitudinal concavity becomes obliterated with the next vertebra. 

 The transverse measurement is from 12 to 13 millim. In the second 

 vertebra the centrum is a little wider and flatter, and it widens 

 posteriorly because the transverse process descends almost to the 

 level of the base of the centrum, and widens transversely in a direc- 

 tion nearly horizontal. In the third vertebra the width is nearly 

 15 millim. ; in the fourth it is 16 millim., and in the fifth and 

 sixth it is 13 millim. 



The first transverse process (fig. 3, t) is on the neural arch ; but in 

 the succeeding vertebras they descend, till the base of the fourth pro- 

 cess is on a level with the base of the centrum : their antero-posterior 

 extent gradually increases as they extend backward. The sixth 

 vertebra is at first less conspicuous than the others, because its 

 transverse process does not descend to the base of the centrum, from 

 which it is defined by an emargination. All the transverse pro- 

 cesses, except the last, are broken (fig. 2), and the measurement across 

 them and the intervening vertebra, as preserved, in no case exceeds 

 3 centim. ; but the abrasion is probably slight, since the processes 

 are as long as in many existing birds, and in some cases the corre- 

 spondence with birds in contour and length of the processes is 

 absolute. 



The side of the sacrum below the neural platform and between 

 the transverse processes appears in all cases to be excavated. The 

 transverse process between the first and second vertebras is vertical, 

 15 millim. high and 6 millim. from front to back. The other 

 processes have some resemblances with Megalosaurus, but appear to be 

 vertical (fig. 3, t) and constricted in the middle, except the last, which 

 is sub-ovate, but little elevated, 12 millim. high and 10 millim. wide. 



The neural canal is wider than high (figs. 4, 5). 



The only extinct animals with which this type of sacrum can be 

 compared are Dinosaurs and Ornithosaurs. With described Dino- 

 saurs, notwithstanding many general points of resemblance, it has 

 comparatively little in common. jNIo genus approaches it more 

 nearly than Hypsilophodon, in which the character of the sacrum is 

 altogether different in the transverse processes of the first sacral 

 being given off on a level with the base of the centrum, while in the 

 later vertebras these processes originate higher up on the sides of the 

 centrum. Mr. Hulke has described a shallow median groove as 

 occupying the ventral surface of the centrums of the sacrum in that 

 genus ; but it is very different in character from the vascular groove 



