OCR Text |
Show 1895.] HYOID BONE OE CEBTAIN PABBOTS. 165 of the bony isthmus lies, as it were, at the bottom of a bony valley formed by the much inwardly inclined dorsal surfaces of the rest of the two entoglossals. Each entoglossal presents a dorsal surface which is slightly concave from within outwards and looks upwards and inwards and is much curved, convex dorsad, antero-posteriorly, especially at and behind the bony isthmus, the part posterior to which has a dorsal surface convex in both directions. The ventral surface of each entoglossal is correspondingly inclined downwards and outwards and is antero-posteriorly concave, while it is slightly convex dorso-ventrally afc its anterior portion, the hinder portion being dorso-ventrally concave. Each entoglossal expands slightly towards its anterior end, where it shows a tendency to bifurcate; the dorsal surface of this most anterior part is concave, while ventrally it is flattened. Postaxially, its end is somewhat more pointed, but does not bend much ventrad at its point. W h e n seen in profile the dorsal margin of each entoglossal is at first slightly concave, antero-posteriorly, and then strongly convex. Its ventral margiu is nearly straight (with only a rudiment of a ventrad process), till we come to the anterior part of the descending posterior portion of the entoglossal, where it expands dorso-ventrally, bends mesiad, and joins its fellow of the opposite side. It then rapidly narrows to its hinder end, the expanded part being crossed by a slight antero-posterior ridge concave ventrad. Anteriorly each entoglossal hardly diminishes perceptibly in vertical extent and appears truncated at its termination. The posterior side of the bony isthmus, formed by the junction of the entoglossals, presents an articular surface strongly concave from side to side and convex dorso-ventrally. The anterior margin of the bony isthmus is strongly concave from side to side. Each hypobranchial is twice as long as the basihyal. It is stout and much laterally expanded towards its preaxial, articular end, the articular surface of which corresponds with that of the basihyal to which it is applied. At the postaxial end of its preaxial third it contracts rather rapidly, and is slender thence till close to its hinder end, where it expands and articulates with the cerato-branchial. The anterior part of the dorsum of each hypobranchial is antero-posteriorly grooved. Each ceratobranchial is in the form of a small flattened ossicle, a little more than twice as long as broad, strongly bent concave mesiad, not tapering towards its termination, which is tipped with cartilage. LOBIUS DOMICELLA. (Fig. 2, p. 166.) Compared with Psittacus erithacus, as regards the structure of the hyoid, Lorius domicella has the basihyal more elongated and its posterior part less expanded laterally and less quadrate in shape. The dorsal lip of its anterior articular surface is more elevated (so that its summit is visible when the hyoid is viewed in profile) and |