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Show 1 9 0 6 .] TIIE SKULL OF A YOUNG RIBBON-FISH. 5 4 9 The Angulare (ang.) is a very small bone on the inner surface of the infra-meckelian part of the articulare. It does not meet the dentary as the angulare of R. glesne does. B. The Suspensorium and Hyoid Arch. The Quadrate has the usual triangular shape with a curved base directed backwards ; the apex, which is directed forwards, not downwards as in ordinary fish-skulls, articulates with the articulare at the level of Meckel's cartilage. The Ectopterygoid (PI. XXXVIII. figs. 1 & 2) is a small triangular bone lying in a nearly vertical plane. At its lower end the bone is pointed and gradually widens to its upper border, where it meets the palatine. The posterior margin fits on to the anterior upper margin of the quadrate, and the dorsal border continues the curve of that bone (PI. XXXIX. fig. 7). Relatively it is a short bone compared with that of R. glesne, in which it projects for a considerable distance-almost half its total length-beyond the quadrate. Behind the Ectopterygoid lie the Ento- and the Metapterygoid, extending below the orbit and curving in towards the middle line. The entopterygoid stretches from the posterior border of the palatine, pterygoid, and quadrate anteriorly to the hyoman-dibular posteriorly; only the upper margin is ossified, a considerable amount of cartilage still existing along the lower border. The metapterygoid is a very small ossification of oblong shape lying below the hinder end of the entopterygoid. In R. glesne the latter is a somewhat quadrate bone about as high as it is long. In the present species the great length and relatively small vertical height of this bone form a marked contrast, as it extends beyond the metapterygoid nearly to the hyomandibular, whereas in R. glesne the Kjone does not reach the hinder end of the metapterygoid. TheN metapterygoid, too, is longer and narrower, relatively, than in R. glesne, and wholly underlies the entopterygoid. \ . Above the pterygoicklies the Palatine, a V-shaped bone with the sharp apex directed forwards. The two limbs embrace the end of the mesopterygoid. Although this bone in R. glesne is very irregular, there is little indication of tins deep notch. The Hyomandibular articulates with the cranium just behind the orbit and below the pterotic. At this end it is broad and thick, but narrows to a point as it passes downwards and forwards. This narrow portion lies on the inside of the pterygoid plate. The articular end is capped by cartilage, and under this is a convex articulation for the opercular bone. At its anterior, pointed, end the hyomandibular is connected with the Symplectic, which extends to the quadrate, thus connecting it with the hyomandibular. In shape the symplectic is a long rod-like bone, slightly curved and much more elongated than that of R. glesne. |