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, Mackie '—this fossil having been placed with the 
Hybodents 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 
lett 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 vertebree appear at the back. 
The pterygo-quadrate cartilage (figs. 1, 2, pg) 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 fa!ls again to some extent, 
though soon re-ascending in the form of a gentle curve to a still 
higher prominence (0¢), 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 the 
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 car- 
tilage is almost straight for its anterior two thirds, the dentigerous 
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 
sueceeded by a short, wavy articular edge, fitting into the corre- 
sponding 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 frent and behind ; posteriorly the curve is at first 
1 §. J. Mackie, “‘ On a new Species of Hybodus from the Lower Chalk,” 
‘ The Geologist,’ vol. vi. (1868), pp. 241-246, pl. xiii. 
