On the anomodont Reptilia and their allies / by H.G. Seeley.

  • Seeley, H. G. (Harry Govier), 1839-1909.
Date:
1889
Catalogue details

Licence: Public Domain Mark

Credit: On the anomodont Reptilia and their allies / 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.

    17/120 (page 229)
    verselj, with the fracture passing through the internal carotid canal (fig. 2). This canal opens on the cerebral surface by a circular foramen, descends vertically, and forks in an inverted Y-shape, so as to have lateral external openings in front of the middle of the inferior descending hypapophyses. So much of the base of the brain-case as is seen is smooth and flat. It is contracted at the anterior corners by lateral tubercles, which may mark the limit of the cerebellum. Behind them the bones enclose a sub- hemispherical cavity, which is 1 '9 centim. wide and about as high, though the height is probably reduced by crushing. The anterior border of this cerebral chamber is smooth, and indicates a vertical lateral vacuity in the skull. The ex-occipital bones appear to extend upward and forward in front of the vacuity, forming a concave root to the back of the brain, but giving off on each side a lateral process which extends forward beneath the so-called supra-occipital bone. This bone (fig. 1) is small on the occipital surface, is narrow, divided by a vertical suture, and situate high above the foramen magnum, has only a linear longitudinal exposure on the median line of the roof of the skull (fig. 2), and extends forward beneath the spatulate bones, which have commonly been regarded as the parietals. The height of the brain-case where the ex-occipital bones terminate in front is 2'7 centims. And then the (?) supra-occipital bone comes into the roof of the cerebral chamber anteriorly, without increasing its width, so that the transverse measurement of the bones, with the median longi- tudinal interspace which divides them, remains 1'6 centim. The bone forms two distinct parallel plates, which are subtriangular, compressed laterally, and at the anterior fracture are 1‘7 centim. high, wdth the internal surfaces vertical and parallel, and divided by a space 7 millims. wide, which is narrower at the superior border, where a strip of the supra-occipital, 2 millims. wide anteriorly, and 5 millims. wide posteriorly, is exposed. The width of the bones posteriorly a,t the summit of the occipital plate is about 1‘5 centim., increasing suddenly below the median posterior groove to 2’5 centims. Their height above the occipital condyle, posteriorly, exceeds 3 centims. It is impossible not to recall the description of Loxomma given by Dr. Embleton and Mr Atthey* in relation to these bones. They remark, “ The upper border of the occipital surface is also the posterior border of the middle ]mrt of the skull.” “ It is formed externally by the mastoids, and between them by a pair of bones corresponding to those which, in Archegosaurus, are called by von Meyer . . . supra- occipitals. Immediately below this border runs a transverse line of suture connecting the bones forming the border with those beneath it, namely, next the median line with the single, and, as we deem it, the true supra-occipital, and laterally with the ex- occipitals. The supra-occipital is of a subtriangular form, wider from side to side than from above downwards, and situated on the median line. It is doubtful whether or not the median suture passes through it. Below, it articulates with the ex-occipitals.” When the external surface of the specimen B. 1021 is compared with the corre- sponding portion of R. 868 there is a close resemblance, though the latter shows the * ‘ Ann. Mag. Nat. Hist.,’ July, 1874, p. 50.