Dicranozygoma leptoscelus, gen. et sp. nov. / H.G. Seeley.

  • Seeley, H. G. (Harry Govier), 1839-1909.
Date:
1900
Catalogue details

Licence: In copyright

Credit: Dicranozygoma leptoscelus, gen. et sp. nov. / 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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    In its absence some uncertainty attaches to the determination of the aflSnitiesof the animal, although enough of the skull remains to show that there are striking features which separate it from all known genera of Theriodontia. It is one of the most important skeletons hitherto found, and is unique in the evidence which it affords of coordination of characters of the dorsal aspect of the more interest- ing bones. The Head. The skull appears to have been about 4| inches long in the median line; but its greatest length was about 6 inches, owing to the unusual expansion and backward lateral prolongation of the squamosal boues. The greatest width of the skull transversely was in the line of the vertical occipital plate, where the measurement from side to side exceeds 4^ inches. Tbe inner borders of the orbits of the ej^es are concave in length, and the least width of the interspace between them exceeds 1 inch. The hinder lateral border formed by the postfrontal bone is preserved on the leftside, and has a thickened rounded prominent edge. This triangular bone, narrowing as it extends outward, is concave between its prominent anterior and posterior margins, and makes the anterior limit of the short and wide temporal vacuity. The transverse measurement of the postfrontal is about J inch. Its narrow outer edge is directed downward towards the malar region. The space between the postfrontal bones, occupied by the frontal bones, is slightly convex, and is traversed by short elevated longitudinal ridges, which are not parallel but meet so as to enclose long, shallow, fusiform pits, seven or eight in number, in the trans- verse width of the bones. This ornament is extended backward upon the parietal region; but there is no evidence of it farther forward, because the front of the head is in the missing part of tbe specimen. This Labyrinthodont type of ornament is known in certain skulls of Theriodonts, but is unknown in Oitdmodon, Dicynodon, or any other South African reptiles with which this fossil may be compared. I have not seen, however, the same net-like pattern in Cynoynathus, or any Theriodont hitherto described, liehind the postfrontal bones the short parietal region of the skull has slightly concave sides with sharp edges which converge some- what as they extend backward. The parietal foramen is longitu- dinally oblong, and about 1 inch in advance of the occipital plate. Immediately behind it the surface of the parietal bones becomes concave. The occipital plate is nearly vertical, and subtriangular, ^ inch wide at the apex, at the median superior crest of the occiput. This region widens with a narrow concave lateral margin, which e.vcavates a deep wide notch between this triangular jdate of the skull and the lateral expansion of the squamosal bone, correspoiiding to the condition of the narrow edge in the triangular occipital plate of Gomphognathus, and corresponding to the deep lateral V-sha])ed notch seen in that genua between that jilate and the squamosal region of the zygomatie arch. The width of the base of the occipital