306
D. B. NORMAN
Figure 2. PIN 559-1/1. Holotype I. orientalis. Right maxilla, lateral view. Even tone = plaster of Paris; cross-hatching = broken surface. Abbreviations: ap, median anterior process of the maxilla; ect, ectopterygoid suture; jug. p, jugal process (broken away); la, contact area for lacrimal; lap, lateral anterior process; rug, rugosities for attachment of premaxilla; tr, trough to receive the posterior lateral process of the premaxilla.
is an arcade of small foramina (Fig. 3, f) each one corresponding to a vertical tooth family; these are by convention interpreted (Edmund, 1957) as foramina for the entry of nutritive blood vessels and sensory nerves to the dental lamina. All the foramina are linked together by a shallow groove which presumably marks the path taken by the main blood vessel and nerve trunk supplying these foramina. The area between these nutritive foramina and the alveolar margin forms a thin alveolar wall or parapet (Fig. 3, al.p), which supports the roots of functional teeth and the crowns of successional teeth. The height of the alveolar parapet is greatest just caudal of the mid-point along the upper jaw and it is here that the largest teeth emerge; the surface of this parapet, which has a fine irregular ornament, is totally unlike that of the rest of the maxilla. This bone structure presumably relates to the metabolic needs of this region; the rapid replacement of teeth, which occurred in these animals, implies a rapid turnover of minerals in the alveolar bone for the continuous remodelling which must have occurred as the teeth grew. Rostral to the tooth row there is an incomplete, tapering process which extends ventrally, curving beneath the area originally occupied by the lateral margin of the premaxilla (Fig. 2, lap) Along the mediodorsal edge of this process the bone is markedly rugose, marking an area for attachment of ligaments binding the premaxilla to the maxilla. Laterally, this surface slopes away to form a smoothly rounded lateral surface. Above the first few tooth positions, the dorsal edge of this process is compressed and forms a relatively thin vertical wall which has a thicker and rugose dorsal edge. The anterior part of this dorsal edge projects forwards as a distinct finger-like process, which bears distinct facets laterally and medially (Figs 2,3, rug). The rugose facets on the medial edge of this rostral process are probably for articulation against the median, posterior premaxillary process (evidence for which can be seen in I. atherfieldensis - Norman, 1986). Caudally, this dorsal edge runs roughly horizontally and merges with the main body of the maxilla — though this area is broken and crushed. Between the vertical septum which supports the dorsal edge, and the lateral surface of the maxilla, there is an obliquely inclined, elongate trough (Fig. 2, tr), somewhat crushed and distorted in this specimen, into which would have fitted the lateral portion of the posterior premaxillary process. Caudally the groove in the upper surface of the maxilla is interrupted by breakage in the dorsal