MORPHOLOGICAL REVISION 103 



behind. Below the latter they extend as a rather narrow prolongation to or nearly 

 to the hind angle of the skull, and to the outer extremity of the ' epiotic ' bones. In 

 the skull figured by Case and myself these posterior prolongations appear to be 

 suturally separated from the broader part of the jugals in advance. A careful 

 examination of other specimens, however, reveals no suture here and leads me to 

 the belief that the supposed suture is merely a fracture in the same place on each side, 

 due doubtless to the fact of the subangular narrowing of the jugal at this place. 

 If there be a distinct bone here I suppose that it must be the real quadratojugal 

 notwithstanding it has no articulation with the quadrate. All the sutures I have 

 so far described, save perhaps that between the nasal and lachrymal, and that 

 between the postorbital and jugal, are decisively and clearly indicated in the dif- 

 ferent specimens, some of them conspicuously so, and they, moreover, agree in 

 the different specimens, as long and patient examinations and careful measurements 

 testify. 



"Cope's determinations of the cranial elements in Parwtichus and both Case's 

 and my own in the small skull of Labidosaurus recognize another suture dividing 

 the so-called squamosal into two distinct elements, though we do not agree in the 

 position of this suture. In the Labidosaurus skull figured by myself there does 

 appear to be a divisional line, indistinctly shown and agreeing on the two sides 

 pretty well. Unfortunately, in a half-dozen other specimens showing this part of 

 the cranial wall, some of them in the most perfect condition both above and below, 

 I can find no trace of a divisional suture, even under the most careful examination 

 with a lens. I am satisfied that there is none; that there is but a single bone here 

 and not two, and this conclusion was reached before I perceived its significance in 

 comparison with the skull of Dimetrodon. This large, flat and thin, or gently con- 

 vex bone unites on its inner side with the parietal, on the front side with the post- 

 frontal and postorbital, and on the lower or outer side by a very squamous and loose 

 suture with the posterior prolongation of the jugal. This is precisely the arrange- 

 ment of these bones in Dimetrodon, and I am satisfied that the elements are mor- 

 phologically identical. The chief difference between Labidosaurus and Dimetrodon 

 consists in the rather large vacuity of the latter piercing what otherwise would be the 

 squamosal, jugal, and postorbital bones. For the present I accept Case's determina- 

 tion of the squamosal element as the prosquamosal, but I feel far less assured of its 

 homology than I did formerly, though I doubt not that it corresponds quite with the 

 element in the ichthyosaurs originally named prosquamosal by Owen. 



"On the posterior or occipital side there are two cranial roof bones on each 

 side, clearly and positively shown in all our specimens, one bordering the hind margin 

 of the parietal, the other the squamosal, and called by Cope respectively the supra- 

 occipital and the tabulare — that is, the so-called epiotic of authors. They differ 

 from the bones of the upper surface of the skull in lacking the superficial markings 

 or pittings, and are suturally united with the superior bones at an angle of nearly 

 ninety degrees. The superior or inner of these two pairs of bones, those bordering 

 the parietals, the supraoccipitals of Cope, are the narrower of the two. Their inner 

 ends are curved downward slightly, with an angular interval between them, into 

 which fitted the small spine of the real supraoccipital described further on. It has 

 long been believed that the so-called supraoccipital of the Stegocephala and of those 

 reptiles in which a like bone is believed to occur does not correspond to the true 

 supraoccipital of the higher reptiles and mammals. They are clearly membrane 

 bones, and have been called the postparietals by Broom. That they are not the 

 real supraoccipitals is very evident in this specimen, in which a large and well-defined 

 supraoccipital is found quite dissociated from the membrane bones of the cranial 



