On the primitive reptile Procolophon / by H.G. Seeley.

  • Harry Govier Seeley
Date:
1905
Catalogue details

Licence: In copyright

Credit: On the primitive reptile Procolophon / by H.G. Seeley. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    Quadrato-squamosal Arch.—The squamosal bone, which occupies a small area on the lateral aspect of the skull between the post- squamosal above and the quadrate bone below, is better seen on the occipital aspect (text-fig. 32), where it forms the upper and narrower part of the quadrato-squamosal pedicle for articulation with the mandible. The pedicle is nearly vertical, being inclined slightly backward as it extends downward, is convex on the straight side, and concave on the outer part, where the portion regarded hitherto as the quadrato-jugal is prolonged behind this surface out- ward and backward. The posterior aspect of the pedicle is crossed obliquely in its middle part by the sagittate suture which divides the squamosal bone from the quadrate, so that, passing downward and inward, it does not reach the mandibular articulation, which is formed by the quadrate bone. The height to the roof of the skull is yp0- inch. The transverse width of the quadrate bone at the articulation is about half an inch. This is exclusive of the great internal process of quadrate contour which extends inward and forward above the infra-quadrate process of the pterygoid bone, and internal to the descending process of the squamosal, and is exposed in one skull by removing the occipital bones. The occipital surface of the skull, properly so called, is entirely behind the squamoso-quadrate region. Its vertical measurement is about half an inch, and the transverse width about an inch and a quarter. Below the postsquamosal bones its contour inclines to be transversely fusiform, owing to the inferior median basi- occipital convexity and the lateral concave inferior emargi nation below the opisthotic bones (text-fig. 32). The foramen magnum occupies the middle of the area. It is higher than wide, wider below than above, margined laterally by an elevated rounded border, such as might possibly have carried a pro-atlas. Inferiorly this border merges in the occipital condyles, which are defined by a median concavity. The sutures are not distinct, but the basioccipital appears to enter into this median concavity, so that the two condyles from which the bony tissue has been rubbed are upon the exoccipital bones. Above the condyles a transverse horizontal suture separates the exoccipital from the supraoccipital bones, which are larger. Externally these bones are limited by a vertical suture, which separates them from the opisthotic, which is subtriangular and terminates outward m a. blunt process below the postsquamosal and slightly in advance of it. There may be an interparietal above the supraoccipital bones and below the parietal. The flattened surfaces of these bones appear a little concave, owing to the elevation of the border of the foramen magnum. The distinctive character of this region is the closed occiput, which is more like that of Crocodilus than lestudo, and if the quadrate bones of a Crocodile were directed downward instead of backward, the occipital region of the skull would be more closely comparable with Procolophon in its back- w.ud extension and elevation above the mandibular articulation. ie only South Afiican reptile which approximates to this
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