14 POSTERIOR CRANIAL ARCHES 



are not exclusively tegumentary, but are ideDtical in character with the bones of the 

 brain case, and the sutures are visible on the under as well as the upper side. 



Pantylus* agrees with Chilonyx in the composition of its cranial roof with 

 the exception that the suspensorium is vertical and is not directed forwards. The 

 position of the supraoccipital and intercalare cannot be ascertained, owing to the 

 condition of the specimens (Fig. 4, Plate I). 



Pariotichus Copef agrees in the main with Chilonyx, but the supraoccipital is 

 divided medially and is reflected onto the superior face of the skull as in Stegocephali. 

 The intercalare is reduced to a small element, of which a small part appears on the 

 superior face of the skull immediately behind the exterior part of the supramastoid 

 (Fig. 3, Plate I). 



It is then these three genera which, according to Baur's theory, represent the 

 type from which the reptiles with posterior cranial bars have been derived by the 

 development of foramina in the temporal roof. Let us see how this has been accom- 

 plished in the different types. I commence with the Permian genera. 



The Theriodont genus, from the Permian of which 1 possess the best preserved 

 cranium, is I!^aosaurus Cope (Fig. 7). It is, unfortunately for our purpose, the 

 most extremely modified. The orbit is in the posterior part of the skull, and the 

 muzzle is greatly elevated and compressed. The zygomatic (quadratojugal) is 

 greatly decurved posteriorly, and the supratemporal is accordingly decurved also. 

 The postfrontal (Fig. 7a) is a narrow bone, wider than long, and it has connection 

 with the frontal, parietal and postorbital only. The postorbital is an L-shaped struc- 

 ture, of which the shorter limb is inferior, extending to the jugal, while the longer 

 limb is posterior, extending to the supratemporal, in contact with the parietal. It 

 encloses no foramen with the latter; but it encloses a larger foramen with the jugal, 

 zygomatic and supratemporal at the other boundaries. This is the infratemporal 

 foramen of Baur. Posterior to the parietal is a small transverse element, which appears 

 to be merely adherent to the former. Its determination is not easy at present. The 

 supratemporal is elongate vertically, and narrow anteroposteriorly. Beneath and 

 towards the middle line of the skull is a part of another bone, which may be the 

 paroccipital, or even exoccipital. The pineal foramen is distinct. No parietoquad- 

 rate arch. 



In ClepsydropsJ the structure is apparently the same, although the form is 

 much less modified. The quadrate articulation is nearly in line with the maxillary 

 dental series (Fig. G, Plate II), and the jugal is nearly horizontal ; its inferior border 



*Cope, Bullet. U. S. C4eol. Survey Terrs., 1881 (8vo). 

 t Proceeds. Amer. Philos. Soc., 1878, ]\ 508. 

 t Cope, loc. cit. , 1878, p. 509. 



