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Show 220 MR. A. S. WOODWARD ON THE MANDIBULAR [Apr. 20, obtained from the Chalk of Kent, and is in an excellent state of preservation. It agrees in all important respects with a less perfect specimen in the same collection, which forms the type of Hybodus dubrisiensis, Mackie1-this fossil having been placed with the Hybodonts in the Museum series by Mr. William Davies, and subsequently figured by the editor of the « Geologist' under the name just quoted. And it may be added that the teeth are undis-tinguishable in general character from those of the typical species of Hybodus of earlier date. The specimen (Plate XX.) exhibits the pterygo-quadrate and Meckelian cartilages of each side,*those on the right, however, being broken away for about the hinder third ; on the left side, the two elements of the hyoid arch are well shown in their natural position, and less complete remains of the corresponding cartilages are also seen on the right. Teeth and shagreen granules are abundant, and three well-calcified vertebrae appear at the back. The pterygo-quadrate cartilage (figs. 1, 2, pq) measures 0*073 m. in length, and varies considerably in depth at different points. At the anterior end it is comparatively low and somewhat folded inwards above, but it soon begins to deepen by the rising of the upper border, which rapidly ascends to form a pterygo-trabecular process (p) ; this is placed at about one third of the distance from the fore to the hinder extremity. The process is almost pointed, and posteriorly the upper contour at once falls again to some extent, though soon re-ascending in the form of a gentle curve to a still higher prominence (ot), which exhibits in front a very distinct, relatively broad articular facette. This occurs at about the end of the second third of the cartilage, and thus far the characters are shown on both the left and right sides of the fossil; but the hindermost third is only preserved to the left. A little beyond tbe summit of the facette, the upper border becomes thickened and turned outwards, and its edge forms a regular gentle curve down to the articulation of the lower jaw. The inferior border of the cartilage is almost straight for its anterior two thirds, the deutigerous portion, and then there follows a curved hollowing ; next is a minute process, marking the commencement of the articular portion, which shows two small hollowings and a terminal convex condyle. The Meckelian cartilage, or lower jaw (mk), is preserved on both sides of the fossil, but is only perfect on the left. It measures 0-0685 m. in total length, being thus somewhat shorter than the pterygo-quadrate. The upper contour is almost straight from end to end, only a slight hollowing being perceptible immediately beyond the termination of the tooth-bearing portion ; and this is succeeded by a short, wavy articular edge, fitting into the corresponding irregularities at the hinder end of the pterygo-quadrate, and finally presenting a distinct concavity for the reception of the upper condyle. The cartilage is deepest just beneath the end of the dentigerous portion, from which point the lower border curves upwards both in front and behind ; posteriorly the curve is at first 1 S. J. Mackie, " O n a new Species of Hybodus from the Lower Chalk " < The Geologist,' vol. vi. (1863), pp. 241-240, pi. xiii. |