OCR Text |
Show 548 PROF. W. B. BENHAM AND MR. W. J. DUNBAR ON [May 15, being almost equal in length ; whereas in the present species the nasal process is more than twice the length of the alveolar plate. Again, the longer axis of this plate is, in R. glesne, vertical and perpendicular to the nasal process, while in R. 'parkeri the longer axis of the plate is almost horizontal and parallel to the nasal process (PI. XXXVIII. fig. 1). The Maxilla, instead of being, as in a typical teleostean skull, a narrow rod of bone, is broad and subquadrate, marked on its outer surface by ridges-as in other dermal bones of Regalecus- which rise from a point near the dorsal posterior border (fig. 1). The maxilla overlaps the hinder part of the alveolar plate of the premaxilla, and can be distinctly seen through the silvery epidermis. On its inner side is a pronounced ridge, which is continued beyond the posterior margin of the bone as a peg-like process (mx.') lying alongside the nasal process of the premaxilla (PI. XXXVIII. fig. 1 and PI. XXXIX. fig. 3). The anteroposterior length is somewhat greater than the vertical height, whereas in R. glesne the bone is long and narrow, and is at least twice as high dorso-ventrally as it is wide : in fact its relation to mouth is more like that commonly met with in Teleosteans. The lower jaw (PI. XXXVIII. figs. 1, 2) consists of the usual three bones, the Dentary, the Articulare, and the Angulare, enclosing Meckel's cartilage, which is distinctly visible through them. The region above Meckel's cartilage maybe termed the supra-meckelian, and the part below that line the infra-meckelian region. In R. glesne, Prof. Parker describes the supra-meckelian part of the lower jaw as having " something the form of an equilateral triangle and the infra-meckelian of a right-angled triangle with altitude about one-fourth of its base, so that the whole jaw comes to be rather higher than long." In the present species the height of the lower jaw is very much greater than the length, and the proportions of the two regions are different from R. glesne. The supra-meckelian portion has the form of an isosceles right triangle with one limb of the dentary as hypotenuse. The infra-meckelian portion is an irregular four-sided figure whose height is one half its length ; it is thus just twice as high proportionately as that of Regalecus glesne (PI. XXXVIII. fig. 2). The Articulare (ar.) is a thin plate of bone somewhat triangular in form. The posterior side is vertical and extends upwards as far as the peg of the maxilla. This posterior margin is much thickened. The lower margin is also slightly thickened, and extends horizontally below Meckel's cartilage, overlapping and concealing the angulare externally. The third side slopes downwards and forwards and meets the dentary along the edge. The Dentary (d.) is a V-shaped bone placed with the angle forwards and bearing at the extremity one tooth which is not present in R. glesne. The two limbs meet one another at an angle of 90°, one being directed backwards and upwards to meet the articulare above, the other passing below Meckel's cartilage to meet the low'er border of the articulare. |