| OCR Text |
Show 1887.] TELEOSTEAN GENUS RHACOLEPIS. 537 maxilla?, and dentaries are likewise armed with a moderately powerful series. These are elongated cones, closely approximated, and varying but little in size, though those on the palatines are apparently the largest. The cleft of the mouth is slightly turned upwards, and the lower jaw scarcely projects beyond the upper; and of the two elements entering into the bony margin above, the maxilla? have much the greater extent. But the most striking feature in the head is presented by the ring of circumorbital bones, which attain to an extraordinary size (Plate XLVI. figs. 1, 2, 5 ; Plate XLVII. figs. 4, 5). Two, or perhaps three, of considerable dimensions are situated behind the eye, while the largest occupies a postero-inferior position ; and the anteriorly directed process of the latter, which is very narrow in R. buccalis, bounds the orbit below, in conjunction with the small foremost element of the ring. Posterior to this series, the long narrow preoperculum is seen ; and still beyond are the other elements of the opercular fold. The operculum itself varies in form in the different species (compare Plate XLVI. figs. 1, 2; Plate XLVII. figs. 4, 5 ); the suboperculum is relatively large ; and there is a distinct triangular interoperculum. None of these bones exhibit any definite ornament, sculpturing, or marginal denticulation, and there was no extension of the scales over any part of the cephalic region. The branchiostegal rays attached to the epihyal are about ten in number, aud large aud much expanded ; but in front of these the ceratohyal supports an apparently equal series, which are quite of small size (Plate XLVII. fig. 1), and likewise differ in being considerably "spaced out." The vertebral column is not completely shown in any specimen, and it is thus only possible to determine the number of vertebrae approximately : in R. buccalis there appear to be not less than 24 in the abdominal region, and perhaps as many as 20 in the caudal. The centra are well ossified, though perforated in the middle for the passage of a remnant of the notochord ; and they exhibit no large lateral excavations, but are marked by delicate longitudinally extended pittings (Plate XLVI. fig. 4 a). The ribs are tolerably massive, as shown by B. M . no. 47900, and the same fossil displays a considerable number of crushed intermuscular bones. Both neural and haemal arches in the caudal region are remarkably strong and elongate; but it is unfortunately impossible to determine with certainty the modifications for the support of the tail-fin. In the pectoral arch the form of the clavicle is well shown, especially by one small fragment (no. P 1958 c). It has a gentle longitudinal curve, the concavity being anterior; and there is a comparatively broad, inwardly directed wing, in addition to the exposed part in the plane of the side of the body. The pectoral fin is robust (Plate XLVI. fig. 1), and the several stout rays are divided distally ; but it is impossible to determine the exact number of these rays, though there cannot have been less than 18 or 20 in R. buccalis. The "pelvic" bones are only well shown in one specimen, belonging to the small species just mentioned, and the element of the right side |
