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FOSSIL REPTILIA OF THE 



the Wealden and Purbeck,' Part IV, Palseontographical Society's Volume for 1856. 

 In the sacrum of the HylcBosaurus there figured the vertebra No. 5 offers the greatest 

 breadth and flattening of the under surface, which is also notable for the absence of the 

 longitudinal ridges, parial or single, marking the under surface of the succeeding or pre- 

 ceding centrums. 



The under surface of the present sacral (PI. Ill, fig. 1) is less accentuated than the 

 Hylaeosaurian one compared with it, and the venous canals are relatively smaller than in 

 it : they also issue irregularly, instead of being symmetrically disposed as are the large 

 pair in Hylaosaurus. The under surface, as shown in the side view (ib., fig. 4, c), is feebly 

 undulate lengthwise, the concave curves being mainly due to the expansion of the articular 

 ends (ib., fig. 3). The under surface of the centrum is as moderately convex across, 

 becoming flat near the free portions of the side of the centrum, (ib., figs. 1, 2, 4, c') 

 and very slightly concave through the distal expansion of the parapophyses (ib., fig. 1, 

 p p 1 ). But the distinctive peculiarity of the present centrum from the known sacral ones 

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

 ( c ') between the origins of the parapophyses ( Pt p ') into a long, low and deep cavity (ib., 

 figs. 1 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, n), 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 

 the broken bases of the neurapophyses (np) which have not extended so near to the 

 end b, as to the opposite end, a, of the centrum. 1 



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, d). It may be that the approximation of the roof and floor of the lateral 

 fossae has been increased by pressure. Yet the horizontal surface, /, 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, further, 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 



1 Compare the figure of the sacral vertebra of Iguanodon, PI. VII, fig. 4, o, o, in the 'Monograph on 

 Reptilia of the Wealden/ Part II, in the Palseontographical Society's volume issued for 1854. 



