346 BRITISH FOSSIL REPTILES. 



Tlie tibia. Plate 31. 



The specimen, from which the reduced figures have been taken in the above 

 plate, is the most perfect one of the tibia of the Megalosaurus which has hitherto 

 come under my notice : it originally formed part of the collection of Megalosaurian 

 remains from the Stonesfield slate, acquired by the Earl of Enniskillen, whilst an 

 undergraduate at Oxford, and is now in the British Museum. 



Fig. 1 gives a side view of the bone, with a top view of the upper articular surface. 

 The divisions corresponding with the condyles of the femur project from the back 

 part of the proximal end, which gradually contracts towards the fore-part where it 

 assumes the character of a process, answering to the procnemial ridge in the tibia of 

 birds, but it is a little inclined inward. The articular surface is a little concave at its 

 middle part and becomes convex, in a moderate degree, upon the condyles. A thick 

 cartilage appears to have covered the whole of this surface, and the softer bone in 

 contact with the cartilage has been, as in most fossil reptilian long bones, more or less 

 abraded, especially at the margins of the articulation. The backward position and 

 production of the corresponding articular prominences or condyles in both femur 

 and tibia, indicate that these bones were joined together at ati angle, probably 

 appi'oaching a right one, when in their intermediate state between flexion and 

 extension : and that motion of the tibia in the latter direction could not have taken 

 place to the extent required to bring the two bones in the same line. A moderately 

 developed longitudinal ridge, fig. 2, c, extends from the inner side of the upper fourth 

 of the shaft of the tibia, the homologue of which is present in the tibia of the great 

 Monitor. Below this the shaft of the tibia assumes a sub-trihedral figure, with the 

 angles unequally rounded off, fig. 3 ; it very gradually decreases in breadth, from before 

 backwards, to within a short distance of the lower end : the transverse diameter 

 remains the same. The expansion of the lower articular end is chiefly in the latter 

 direction, i. e., at right angles with the long diameter of the proximal end : the inner 

 angle of the distal end is the most produced. The form of the articular surface for 

 the tarsus is a rhomboid, with two shallow depressions, but in the main is moderately 

 convex. 



The length of the bone above described is 26 inches : its shaft, like that of the 

 femur, has a medullary cavity, but the compact walls are relatively thicker in the 

 tibia. 



The above- described bone, from the Oolitic slate of Stonesfield, presents all the 

 main Dinosaurian characters, which have been described, in a preceding section, 

 in the tibia of the I(/uanodon.* From that tibia the present bone differs in its 



* P. 31.3. 



