40 PERMO-CARBONIFEROUS VERTEBRATES FROM NEW MEXICO. 



erately broad jugals. Above, the orbits are roofed over by a projecting plate from 

 the frontals. The opening of each orbit, as preserved, looks laterally and not at 

 all upward, and this must have been essentially the condition in life. The pos- 

 terior border, formed by the postorbital above and the jugal below, is thickened, 

 as is also the antero-superior border. Below, in front, the margin is very thin, 

 and the precise edge here has been lost in preparation. Immediately back of the 

 upper, dilated, part of the orbit, and separated by a narrow but firm bar, is the 

 relatively small, oval, lateral temporal fenestra, an opening 25 mm. in its greater, 

 13 mm. in its conjugate diameter, the former at an angle of about 45 degrees with 

 the condylar-narial axis of the skull. Its postero-inferior margin is thin; elsewhere 

 the border is thicker and rounded. Above, this vacuity is separated by a thick, 

 subcylindric or oval bar from a smaller, oval vacuity or foramen, the supratem- 

 poral fenestra, which is about 12 mm. in its greater diameter at the bottom of a 

 fossa, which is partially over- roofed by the posterior prolongation of the parietal. 

 There seems to be no doubt that this opening is a normal one, not due to some 

 accidental separation of contiguous bones, since it is alike on the two sides of the 

 skull. On each side the clay matrix filling the fossa and vacuity was removed with 

 care, proving conclusively that the thick, rounded, smooth, and deep margins 

 were everywhere normal, without signs of sutural roughening or fractured surfaces. 

 This opening, bounded below by the thickened supratemporal arcade, which 

 evidently is composed of the united postorbital and squamosal, posteriorly by what 

 is certainly the squamoso-parietal bar, and superiorly by the parietal, can be inter- 

 preted as nothing else than a true supratemporal fenestra. 



Below and back of these vacuities there is a broad expanse of thin bone, 

 doubtless composed of a squamosal element, the jugal and the quadratojugal. The 

 striations on the clean surface of the bones in this region, as also the position of 

 lines which seem to be sutural, indicate their relative extent and articulations 

 about as they are shown in the figure. With these interpretations, the jugal in 

 front forms only a small part of the free lower border of the maxillo-quadrate arch. 

 A sutural line seems to distinguish the jugal from the post-orbital above, as shown 

 in the figure between the orbit and lateral fenestra. Posteriorly the diverse stria- 

 tion and an intervening sutural line distinguish the jugal from an element that 

 is doubtless the true quadratojugal, which extends far forward on the lower, free 

 border of the arch. The position of this suture is very different from that bound- 

 ing the jugal behind, positively and definitely known in Dimetrodon. In this genus 

 there is but one bone articulating with the jugal posteriorly, the prosquamosal of 

 Case, the squamosal of authors; at least neither of us is convinced of the presence 

 of a lower bone articulating with the jugal, which if present must have joined its 

 extreme tip only, taking no part whatever in the margin of the temporal fenestra. 



Case believes that there is no lateral quadratojugal bone in Dimetrodon, but 

 that the bone properly so-called is attached to the posterior side of the quadrate, 

 extending upward. A recent examination of the structure of Dimetrodon in this 

 region by Williston in material that is, perhaps, as good as any known, tends 

 to convince him of the general correctness of Case's observations, though he is not 

 at all assured of the correctness of his interpretations. The squamoso-jugal suture 

 in Dimetrodon passes backward obliquely from above downward to the extreme 

 tip of the jugal. Back of the jugal, covering the quadrate, there is an element 

 which seems to articulate with an upper bone lying posterior to that identified as 

 the prosquamosal by Case. Whether or not this bone touched the extreme tip of 

 the jugal Williston can not decide, but he thinks not. Between it and the quadrate 

 posteriorly there is a distinct foramen, identified as the quadrate foramen by Case. 



