MORPHOLOGICAL REVISION 95 



typical Stegocephalian skull, comes not only the recognition that these forms can 

 not be the primitive stem groups of the reptilia but also the serious morphological 

 question of the homology of the bones. Is the lower bone of the two covering the 

 temporal region and connecting the quadrate with the jugal, the quadratojugal of 

 higher Reptilia or is it the prosquamosal ? In Dimetrodon there is a single plate on 

 the posterior side of the quadrate, connecting with the postorbital above and partially 

 separated from the quadrate below by a quadrate foramen. This bone I have 

 called the quadratojugal, and the bone which connects it with the jugal I have des- 

 ignated as the prosquamosal (16). This nomenclature is quite different from that 

 usually accepted and several objections have been urged against it. Notably, that 

 the paroccipital (opisthotic) comes in contact with the bone called quadratojugal, 

 which is not common among recent reptiles, and that the second bone should not be 

 called prosquamosal, because the name quadratojugal has commonly been applied 

 to the element connecting the quadrate with the j'ugal. These objections have been 

 partially replied to and the arguments, somewhat amplified, are repeated here. 



In the first place it must be clearly recognized that: (i) The Rhyncocephalian 

 skull can not be regarded as a stem form from which all other reptilian two-arched 

 types were derived; (2) the Rhyncocephalian skull can not be derived simply and 

 directly from the Cotylosaurian skull; (3) the Cotylosaurian skull can not be 

 regarded as the single primitive type of reptilian skull. If we free our minds of 

 these conceptions, so long dominant in the theory of the development of the reptiles, 

 the questions are less difficult. 



The first objection to designating as the quadratojugal the element which 

 covers the quadrate posteriorly and is separated from it in the Pelycosaurs and Dia- 

 dectida by a quadrate foramen, is that it comes in contact with the paroccipital in 

 Labtdosaurus and probably in Captorhinus, Diadectes, and the Pelycosauria also. 

 This objection is not vital, for such a condition occurs in the dinosaurs Allosaurus 

 and Tyranosaurus, where the element (quadratojugal) comes directly in contact 

 with the jugal anteriorly and is unquestionably a quadratojugal; in other respects it 

 is almost exactly the same as in the Pelycosaurs and morphologically the same as in 

 Captorhinus, with the possible exception of the lack of a quadrate foramen in that 

 genus. 



It will be seen that the bones on the posterior surface of the skull in Cap- 

 torhinus and Labidosaurus have essentially the same arrangement zs in Dimetrodon; 

 in the latter genus the cartilaginous supraoccipital has developed, and the need for 

 the dermal supraoccipital plates having passed they have disappeared. This 

 process can be seen in Labtdosaurus^ where both the cartilaginous supraoccipital 

 bone and the dermal supraoccipital plates are present. When the supraoccipital 

 plates disappeared the parietal became the posterior bone on the upper surface 

 of the skull and the squamosals became the posterior elements on the sides of the 

 skull; the tabulate is just disappearing in Captorhinus and is completely lost in 

 Labidosaurus, the Pelycosaurs, and the Diadectida: (?). The squamosals would 

 now have the same relation to the upper end of the bones lying on the posterior 

 face of the quadrate that the supraoccipital plates did in the Cotylosauria. 



There is no inherent improbability that the element connecting the quadrate 

 with the jugal is the prosquamosal (supratemporal); indeed if the arguments offered 

 above have weight, it must be by exclusion. Other facts, however, may be cited 

 in favor of this idea. The temporal region in the Stegocephalia is covered by the 

 parietal, squamosal, prosquamosal, quadratojugal, and jugal. The prosquamo- 

 sal is commonly the largest element of the temporal region; it articulates with the 

 postorbital anteriorly below, and in many forms, as Branchiosaurus, etc., pos- 



