OCR Text |
Show 1890.] UPPER CRETACEOUS FISHES. 635 extremely narrow and deep, the orbit (orb.) relatively large and posteriorly situated, and the surrounding membrane bones well developed. The jaws and facial bones seem to have been almost smooth, the mandible only being marked by a longitudinal series of perforations for a sensory canal; but the cranial roof is ornamented with numerous tuberculations, which are also visible upon the stout preopercular spine. The preoperculum (p.op.) is comparatively robust, triangular in shape, tapering to a point above, and abruptly truncated below ; its postero-inferior spinous process (s.) exceeds in length the maximum width of the bone, is sharply pointed, and distinctly appears to have been hollow. The operculum (op.) and suboperculum are comparatively thin, the former deeper than broad, and the latter broad in proportion to its depth ; the only ornament exhibited consists in a few feeble radiating lines upon the operculum. The branchiostegal rays (br.) are very delicate and apparently numerous. Axial Skeleton of Trunk.-The vertebrae are well ossified, apparently simple double cones, somewhat longer than deep, and about forty in number. The neural and haemal spines are firmly united to their supporting arches, and the ribs in the abdominal region are very robust. There are also numerous short intermuscular bones, in their crushed state transversely overlapping the arches of the axial skeleton. Appendicular Skeleton.-The fin-rays are robust, and in all, except the caudal fin, are undivided for a considerable space above their insertion, though apparently articulated and bifurcating distally. In the caudal fin the rays are closely articulated from a point close to the base. In the dorsal and anal fins each ray is borne by a separate interspinous element, but the arrangement of the fin-supports in the caudal is not distinctly shown. The pelvic fins are relatively small and remote, the space between these and the pectorals being three times as great as that between the same fins and the anal. The rays of the latter, about seventeen in number, scarcely exceed those of the pelvic fins in length, but the dorsal fin is relatively much elevated, with not less than twenty stout rays, and is situated completely in advance of the anal. Squamation.-The scales are only distinctly shown in part in the type specimen, but it seems probable that they formed a continuous covering. They are all very thin, and their most conspicuous markings are the concentric lines of growth, along which a feeble ornament of fine rugae and tuberculations is developed. A single series of deep narrow scales, at least half as deep as the trunk, occupies the flank; and above (probably also below) there are smaller, more nearly equilateral scales, likewise of quadrangular shape. Formation and Locality.-Upper Cretaceous (Upper Senonian); Hakel, Mount Lebanon, Syria. EVOLUTION OF THE ASPIDORHYNCHID_E. The Cretaceous species assigned to Belonostomus are so closely |