| OCR Text |
Show 1894.] SALMONOID FISHES OF THE ENGLISH CHALK. 661 The cheek-plates are large, but comparatively thin ; and the only evidence of ornament consists in radiating lines on the large postero-inferior suborbital (B.M. nos. 49903, P. 5681). The maxilla is very robust and arched so that the oral margin is convex (fig. 4, mx.). Anteriorly it is produced upwards and inwards as a long narrow process (p.) above the premaxilla towards its attachment in the front of the ethmoidal region ; the upper portion is a thin lamina overlapped by two supramaxillary plates (figs. 3 a, 4, s.mx.), of which the hindermost is deepest and has an anteriorly directed process at its front upper angle. The premaxilla (fig. 4, pmx.) is shown to have been moderately extended beneath the maxilla. The mandible is deepest in the corouoid region, and the angular element (fig. 2 a, ag.) is very large, extending for a length of more than two-thirds that of the dentary (d.) ou the outer surface. The inferior border of the ramus is slightly bent inwards, and along the angle thus formed the sensory canal opens in a longitudinal series of small pits (fig. 5). The teeth are very minute and clustered on the margin of both jaws. Behind the occipital on each side there is a small supratemporal plate, and partly covered by this is a large post-temporal element. The operculum (fig. 5, op.) is trapezoidal in form, nearly twice as deep in front as behind. The subopercidum (s.op.) is someAvhat less than half as deep as the latter, and has a prominent ascending process at its antero-superior angle. The preoperculum (fig. 4, p.op.) is well exposed, with the inferior limb sharply bent forwards ; and its outer face is marked by a sharp vertical ridge giving rise to a few radiations at the angle. The interoperculum (fig. 5, i.op.) is long and narrow, and the branchiostegal rays, shown to the number of eight or nine in fig. 5 (br.), are broad. It is also interesting to observe that between the rami of the mandible (fig. 5, d.) there is a long narrow azygous gular plate (gu.), quite smooth on the outer face and regularly rounded at each end. One specimen (no. 47932) exhibits at the back of the head remains of a close series of small styliform bones, acutely pointed at the anterior end, attached by a slight expansion at the hinder end. These are probably gill-rakers. The vertebral centra are robust and the secondary lateral ossifications are in the form of delicate longitudinal ridges. Appearances in nos. 49903 and P. 4247 suggest that there was a perforation in the middle of each centrum allowing for the passage of a persistent thread of notochord; Avhile it is clear that the neural arches throughout and the haemal arches in the caudal region are fused with the supporting centra. Bibs also are seen in the abdominal region, not extending quite to the ventral border. The number of Arertebrae cannot be counted, but there seem to have been not less than twenty in the abdominal region. The anterior abdominal vertebral centra are deeper than long-those most posteriorly at least as long as deep. The pectoral arch is obscure, but the remains, shown from, the inner aspect in fig. 6, exhibit the relatively large elongated supra- |
