| OCR Text |
Show 1°90.] UPPER CRETACEOUS FISHES. 631 and pterygo-quadrate arch are relatively large and expanded laminae, of which the hyomandibular (Plate LV. fig. 3) is the only element well displayed in the fossils underconsideration. The truncated upper extremity of this bone is less than half as broad as its inferior expansion, and a few irregular ridges radiate from the middle of its upper moiety, opposite the point at which the short and stout process (p.) for articulation with the operculum occurs. The entopterygoid is a long, narrow, lenticular bone, adjoining the upper margin of the short metapterygoid and large ectopterygoid elements (no. P. 3810). There is no definite information concerning the mandibular and maxillary bones, and the only teeth to be observed are very minute slender conical cusps, which seem to have been arranged in clustered series upon a hinder bone of the upper jaw and the splenial (nos. 28616 and P. 975 b). Round the eye some very small membrane bones represent a discontinuous or rudimentary circumorbital ring (Plate LIV. fig. 1, co.) ; and two trapezoidal elements of a large suborbital series (s.o.) cover the whole of the space between the circum-orbitals and the preoperculum, while a third irregularly triangular bone adjoins these below. The preoperculum (p.op.) is of very large size, triangular in shape, terminating in a pointed upper extremity almost at the antero-superior angle of the operculum, and gradually expanding downwards and forwards, finally bounded by a long, straight or gently curved inferior margin, well below the level of the suborbital ring; the maximum depth of the bone equals somewhat less than twice the length of this margin. The operculum (op.) is also very large, slightly deeper than its maximum breadth, and nearly flat, though bent inwards above. It is irregularly quadrate in shape, and the postero-superior angle is obliquely truncated, so that its upper border is scarcely two-thirds as long as the lower border. The suboperculum (s.op.) is comparatively small, long and narrow, deepest and truncated in front, and its inferior margin gradually curving upwards to a posterior apex. All the suborbital and opercular bones are ornamented with thick vermiculating rugae of ganoine with short branches, as shown in the illustration (Plate L V . fig. 4) taken from the operculum, the arrangement on this bone and on the expanded inferior portion of the suboperculum being more or less concentric with the borders. Axial Skeleton of Trunk.-Well ossified vertebrae occur throughout both the abdominal and caudal regions. The centra (Plate L V . fig. 5) are in the form of stout double-cones, but they are always perforated by a small thread of persistent notochord. The pedicles of the arches seem to be fused with the centra; and the firmly united neural and haemal spines are very slender, except the haemals at the base of the caudal fin, which are much expanded distally (no. P. 975 d). Ribs have not been observed. Appendicular Skeleton.--The fins are relatively small, and, so far as known, agree precisely with those of the typical Jurassic Belonostomus. The rays are stout, laterally compressed, and unarticulated for a short space from their insertion, but soon become distantly jointed and branched. The more robust portions of the rays are also often coated with smooth ganoine. The pectoral fin (Plate LIV. fig. 1, |
