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Show 1 9 0 6 .] THE SKULL OF A YOUNG RIBBON-FISH. 5 51 The In ter opercular is as much elongated as the preopercular; but is not so broad and also pr esents considerable difference from that of R. glesne. In the latter the posterior margin is curved sharply upwards and the dorsal edge is straight; but in R. parkeri the posterior end of the bone is scarcely turned up and the upper margin is excavated. The markings on this bone originate almost exactly in the centre, whereas in R. glesne they are described as originating " at the junction of the anterior and middle thirds." D. The Cranium. After the removal of the jaws and suspensorium, the form of the cranium, as seen from the side (PI. XXXIX. fig. 6), is somewhat like that of a bird's skull. This appearance is due to its length, to the large orbit, and to the beak-like prenasal rostrum. On the dorsal surface are two median grooves, lying end to end, meeting above the centre of the orbit (PI. XXXIX. fig. 4). The anterior dorsal groove has as its floor a plate of cartilage named the " tegmen cranii " by Parker, while its sides are formed by the frontals. In this groove the nasal process (prnx.') of the premaxilla lies (PI. XXXIX. fig. 3). The posterior groove has as its floor the supraoccipital and as its sides the epiotics and parietals : it is scarcely existent in R. glesne. If the cranium of R. parkeri be compared with that of R. glesne, the most striking difference is the greater length and less vertical height in the present species. This excess of length is mainly due to the greater development of the preorbital region or " beak," which is nearly as long as the orbital region, whereas in R. glesne (PI. XXXIX. fig. 7) it is less than half this proportion. The length of the cranium in the latter species is 11 times the greatest height, but the proportion of length to height in the present fish is 2:1. This difference in relative length is due partly to the greater development of the preorbital region, and in a small degree to the absence of the " subcranial crest" described by Parker, formed by parasphenoid, basi-occipital, and opisthotic. The Occipital Region (PI. XXXIX. figs. 4, 5, 6, 8). The Basi-occipital forms the greater part of the occipital condyle (PI. XXXIX. fig. 8), and is produced forwards and downwards as a median ridge which extends anteriorly to meet the end of the parasphenoid. In contrast to the condition in R. glesne, we may note that it is the most posteriorly placed bone in the skull (PI. XXXIX. fig. 6). Dorsally and anteriorly it is bounded by the exoccipital and opisthothic, ventrally and anteriorly by the parasphenoid. The Exoccipitals entirely bound the foramen magnum (PI. XXXIX. fig. 8), each meeting its fellow below it, so that each takes a small share in the occipital condyle. The greater part of the exoccipital is a posterior vertical plate which extends upwards to meet the epiotic and outwards to meet the pterotic, while it sends |
