OCR Text |
Show 226 MR. H. O. FORBES ON THE [Feb. 28, easy of explanation. The gibbosities, it will be observed, occur n the rostrum over those regions where the vomer does not reach the palatal surface. The removal of the wedge gives more space beneath, causing the premaxillaries to gape, while more anteriorly, in front of the place where the vomer vanishes, the premaxillaries stand still more apart. In the regions where the edges of the premaxillaries are closest the vomer is wedged in on the palatal surface between the bases of the maxillaries, and there the rostral bone, as a rule, grows densest and protrudes furthest above the level of the premaxillaries, and just there often shows no median line or suture. In the male Z. cavirostris the greatest growth of the mesorostral occurs where not only the vomer, but also the lower edges of the premaxillaries, protrude on the palatal surface. The lines or sutures on the surface of the mesorostral are produced by various causes, sometimes (as in the specimen, Plate XIII. figs. 1,1 a, v.s) by the two wings of the vomer meating in the centre \ when the suture may persist or may become lost, according to the amount of squeezing the mesorostral undergoes. Then on each side of the solidified vomer may appear the sutures of the premaxillaries (Plate XIII. fig. 1 a, pmx.s), and very often the thickenings of the interior surface of the premaxillaries (pmx.o) grow up between and shoot above the original petrous walls of these bones, forming another suture, so that there may be as many as five lines traceable on the surface. There may be more if, as sometimes occurs, one of these segments becomes crumpled (Plate XIII. fig. 1, Plate XIV. fig. 5,mr.cr) in the general squeeze of the parts. Hence, as diagnostic characters (cf. M. medilineatus), the lines on the mesorostral bone are also quite valueless. In the most anterior part of the rostrum there is only one median suture (pmx.s), often very well marked, especially in old individuals, where the osseous growths on the interior surfaces of the premaxillaries meet. As has been pointed out both by Sir W . Flower and Sir W. Turner, a suture, or often a deep depression, between the mesethmoid and the mesorostral is generally visible (Plate XII. fig. 1, Plate XIII. fig. 1, meth.s). On comparing the different specimens which I have had an opportunity of examining personally, or by their various published descriptions, the species of the genus Mesoplodon seem to me to be reducible to six :- 1. MESOPLODON BIDENS (Sowerby). Cf. Flower, Trans. Zool. Soc. x. p. 415 (1878). 2. MESOPLODON EUROPCEUS (Gervais). Cf. Flower, Trans. Zool. Soc. x. p. 416 (1878). 1 " It seems probable (as Duvernoy has already pointed out) that the ' central area' indicates the upper extent of the vomer, the only remains of the primitive trough-like cavity being the median slit above and the large fossa behind."-Huxley, Quart, journ. Geol. Soc. vol. xx. p. 394 (1864). |