40 THE HISTORY OF THE PELYCOSAURIA, WITH 



The supraoccipital takes no part in the borders of the foramen magnum, being 

 excluded therefrom by the union of the exoccipitals in the median line above the opening. 

 It is somewhat triangular in outline, with the apex downwards. This is somewhat blunted 

 and rests on the united exoccipitals. The sides join the expanded proximal ends of the 

 paroccipitals. The superior border consists of a broad surface for cartilaginous attach- 

 ment with the parietals. 



The paroccipitals are very broad and massive proximally, but are produced in long 

 distal processes. These processes project at a large angle from the posterior region of the 

 skull and pass obliquely backwards, downwards and outwards. The distal articular sur- 

 face is flat or slightly concave, oval in outline, and probably united to the quadrate by 

 cartilage. The lower surface of these paroccipital processes is marked by two deep pits, 

 separated by a sharp ridge. The proximal portion of the paroccipital is much expanded ; 

 above they are in union with the sides of the supraoccipital and bear at the superior 

 edges winglike expansions for the parietals. Below they are suturally united to the 

 exoccipitals and basioccipital. The lower portions of the proximal ends give rise to two 

 descending flanges already described as joining the basioccipital. The region anterior to 

 this flange is deeply excavated and open, so that the foramen rotundum and ovale are 

 freely exposed. 



Measurements of the posterior cranial region. 



M. 



Distance between distal ends of paroccipitals 0.122 



Distance between ends of paroccipital processes 0.112 



Breadth of foramen magnum 0.014 



Height of foramen magnum 0.009 



Breadth of occipital condyle 0.022 



Height of occipital condyle 0.016 



Distance from top of supraoccipital to lower edge of condyle 0.063 



The basisphenoid (PL I, Figs. 13, 14) is broadly expanded posteriorly and contracts 

 rapidly as it passes forwards, forming a neck just behind the closely approximated faces 

 for the pterygoids, and ends anteriorly in a presphenoidal rostrum. The expanded 

 posterior is marked on its upper edge by a pit, the continuation in the basisphenoid of 

 the Eustachian canal. The centre of the posterior face is prolonged backwards in a spout- 

 like process which lays in the groove described as marking the lower face of the basiocci- 

 pitals. The lower edge of the expanded part is divided by a deep and long notch which 

 ends abruptly anteriorly. 



The basipterygoid processes are near the middle of the basisphenoid. They are 

 short and stout, and in the natural position of the bone were nearly vertical, but directed 

 slightly outwards, downwards and forwards. The upper part of the articular faces is 



