94 HADROSAURUS. 



vex, but toAvards its inferior extremity forms a sloping plane directed obliquely 

 inward, and jutting forward at its termination. 



The inner part of the shaft, Fig. 2, at the upper two-thirds is comparatively 

 narrow and cylindroid. Below, it expands into a broad triangular plane directed 

 somewhat backward, and rendered slightly concave transversely by the prominence 

 of the anterior border, which ends in an angular, roiighened process. 



The outer part of the shaft, Fig. 3, is cylindrical, with a broad, wing-like expan- 

 sion curving backward at its upper extremity, and terminating in a wide, triangular 

 surface at its lower extremity. 



The back part of the shaft. Fig. 4, is cylindroid at the middle, and rapidly 

 expands above into a wide surface rendered transversely concave by the backward 

 projection of the inner and outer portions of the head. At its lower third it rises 

 into an acute ridge separating the inner and outer surfaces of the bone. 



The head of the tibia. Figs. 1-5, in front and at the sides together, forms a semi- 

 circular outline. Its back part viewed from above exhibits three strong prominences. 

 The inner two (Figs. 4, 5), nearly equal in size, form, and direction, constitute a pair 

 of articular condyles separated by a deep notch. The outer prominence is formed 

 by a wing-like expansion of the external part of the shaft, and is separated from the 

 condyles by a wide, concave notch. The articular surface of the head is nearly a 

 horizontal plane at its fore part, but is convex posteriorly as it extends upon the 

 three backward prominences. It is rough and deeply marked Avith vascular grooves 

 proceeding from the back part. 



The tarsal articular surface, Fig. 6, has its long diameter in a reverse direction 

 to that of the head of the bone. In outline it forms an irregular trapezium, with 

 the short anterior side nearly straight and directed obliquely forward and inward, 

 with the inner side gently sigmoid, the outer border deeply sigmoid, and the poste- 

 rior shortest side straight and directed obliquely backward and forward. The sur- 

 face is somewhat rough. In the direction of its long diameter, or antero-posteriorly, 

 it is for the most part concave, but is convex at the back extremity. In the direc- 

 tion of its short diameter it is for the most part convex, but is depressed near the 

 antero-internal angle. 



The interior of the tibia possesses a large medullary cavity. The orifice of the 

 mcduUary nutritious canal is directed downward and is situated at the postero- 

 external part of the shaft just above its middle. 



The measurements of the tibia are as follows: — 



Inches. Lines. 



Length of the tibia externally ........ 36 9 



Length in front ........... 35 



Circumference at narrowest part of the shaft, being just below the 



middle 11 8 



Breadth of the upper extremity . 11 3 



Diameter of middle of Jiead antero-posteriorly ..... 5 6 



Breadth of tarsal extremity. ........ 10 



Diameter antero-internally ......... 5 10 



Diameter at middle of tarsal surface 2 10 



Diameter postero-cxternally 3 4 



