. Bulletin. Natural history; Natuurlijke historie. OSTEOLOGY OF DEINONYCHUS ANTIRRHOPUS 21 absence of a skull table and contacts between the nasal, maxilla and lachrymal, it is not possible to reconstruct the exact form of the snout, but the nasals and lach- rymals indicate that the preorbital portion of the skull was probably distinctly triangular in section, narrow above and broad below. POSTORBITAL This element is represented by two nearly complete bones, a left from skull YPM 5232 and a right from skull YPM 5210, (Fig. 8), plus a fragmentary right. cm FIG. 8. Right postorbital of Deirionyc
. Bulletin. Natural history; Natuurlijke historie. OSTEOLOGY OF DEINONYCHUS ANTIRRHOPUS 21 absence of a skull table and contacts between the nasal, maxilla and lachrymal, it is not possible to reconstruct the exact form of the snout, but the nasals and lach- rymals indicate that the preorbital portion of the skull was probably distinctly triangular in section, narrow above and broad below. POSTORBITAL This element is represented by two nearly complete bones, a left from skull YPM 5232 and a right from skull YPM 5210, (Fig. 8), plus a fragmentary right. cm FIG. 8. Right postorbital of Deirionychus antirrhopus, YPM 5210, in medial (A) and lateral (B) views. Abbreviations: ame—area of origin of the external mandibular adductors; fr—frontal suture; ju—jugal process; sq—squamosal process. postorbital (YPM 5232). This small sample shows some variation, chiefly in the degree of robustness, probably a reflection of age differences. The postorbital is triradiate, with a thin posterior process extending to the squamosal and forming the dorsal limit of the lateral temporal fenestra, a somewhat stouter ventral proc- ess which meets the ascending process of the jugal and a massive and externally rugose, dorsal process that bends medially as a thick, vertical lamina to meet the frontal. The latter contact is in the form of an extensive digitate suture, whereas the posterior process fits into a tapered groove on the external surface of the squa- mosal and the ventral process is joined by a shallow overlapping contact of the jugal dorsal process. Like the lachrymal, the dorsal portion of the postorbital is marked by moderate rugosities or sculpturing. The remaining external surface is quite smooth and flat, but the internal surface bears a prominent and sharply defined vertical ridge which descends from the broad frontal suture down the inner surface of the jugal process toward the contact with that bone. In part, this feature contributed to the posterior wall of the orbit,
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