552 BRITISH FOSSIL REPTILES. 



of other Dinosaurs is the continuation of the free surface, over the side of the centrum 

 (fig. 2, c') between the origins of the parapophyses {p, p') into a long, low and deep cavity 

 (ib., figs. 2 and 4,/,/), overarched by the part of the side of the centrum supporting the 

 neurapophyses (ib., figs. 2 and 4, np), which appear to have been confluent therewith, and 

 to have been removed, with the rest of the neural arch, by fracture. 



This displacement exposes the floor of the neural canal (ib., fig. 2, «)> the breadth of 

 which indicates a sacral enlargement of the myelon, and consequent development of 

 the pair of limbs deriving their nerve-supply therefrom. The issue of a large pair of 

 these nerves is indicated by the continuation of the neural surface outward at o, o, behind 

 tlie broken bases of the neurapophyses {np) whicli have not extended so near to the 

 end d, as to the opposite end, a, of the centrum.* 



Owing to the abrupt continuation of the lateral surface of the centrum into the 

 depressions,/,/, characteristic of the present genus of Dinosaur, the free surface of the 

 side of the centrum presents the form of a smoothly rounded, longitudinally concave, ridge 

 (ib., figs. 2 &4, c'). It may be that the approximation of the roof and tloor of the lateral 

 fossae has been increased by pressure. Yet the horizontal surface, y; could hardly have 

 been bent from the vertical side-surface of the centrum, c', without some fracture of 

 the compact outer layer of bone ; and, farther, if the flat form of the centrum had been 

 due to such cause, the seemingly natural undulate configuration of the under surface, 

 with its expansion at the two ends, would not have been unobliterated and unmodified in 

 the degree exhibited by the fossil specimen. 



The outward production of the fore part of each side of the centrum (fore parapo- 

 physis, p) has a longitudinal extent of an inch and a half, a vertical one at the articular 

 surface of seven to eight lines. The surface is rough and slightly concave ; it may have 

 contributed less than one half of the vertical extent of the sacro-iliac joint at this part. 

 The fractured or roughened surface above this parapophysis indicates a corresponding 

 diapophysial production of the neural arch for extension of the joint. Longitudinally 

 the pre-parapophysial surface slightly inclines toward the front articular siirface, a, of the 

 centrum. This surfiice is flat, very rough, and irregular, indicative of having been 

 broken away from a partial confluence with the opposed surface of a contiguous sacral 

 clement ; the lower part showing here and there a smoothness as of the original free sur- 

 face of this end of the centrum. Above this surface large unossified vacuities are shown 

 in the cancellous texture of the bone. The vertical diameter of the articular end of the 

 centrum is one inch three lines ; the transverse diameter is three inches six lines. The 

 lower margin is not entire, but has been eroded or worn away for an equable extent of 

 about four lines ; along the transverse curve it has not been broken ofl' that end of the 

 centrum. 



The post-parapophyses (p') are shorter antero-posteriorly, thicker vertically ; and the 

 articular surfaces of tliis pair converge at a greater angle to the posterior surface, b, of the 



* Compare the figure of the sacral vertebra of lyiuinoilun, PI. 12, fig. 4, o, o, p, 288. 



