OCR Text |
Show 222 MR. H. O. FORBES ON THE [Feb. 28, their earliest appearance the changes that take place in the rostral bones. Commencing with the youngest specimen I have yet examined, that lettered A above (see fig. 1, p. 221), sections of the rostrum taken at 11, 6|, and 5| inches from its apex show the following appearances :- The vomer appears in the most posterior section (i. e. in that at 11 inches) as a more or less uniform semicylindric spout, with a thick rounded keel, whose sides thin upwards and articulate with a diverticulum of the maxillary (as seen in Clymenia, and more markedly in Physeter) and the sides of the premaxillaries. The premaxillaries are roughly rectangular plates dropped into the maxillaries, and they partly roof-in the trough, their sides entering into the formation of the rostral groove. In the middle section the upward arms of the vomer become smaller; the spout is still floored by the vomer, but its wings are very thin and their walls blend to form a continuous smooth surface with those of the premaxillaries, whose sides also are very thin and stand somewhat more erect (Plate XIV. fig. 2 6). On the outside the premaxillaries articulate with the maxillaries. Still more anteriorly (Plate X I V . fig. 2 a) there is a slight change in the form of the vomerine keel, and the premaxillaries appear on the palatal surface, and prevent any articulation between the maxillaries and the vomer. The vomerine trough in the macerated skeleton is quite empty and very smooth, and in the recent state it is filled with cartilage. The mesethmoid only just enters the posterior end of the groove, between the wings of the vomer. The next older specimen (B) I take to be of greater age than that of the skeleton in the Royal College of Surgeons, figured by Sir William Flower, and about equal (judging alone by the figure, plate lxii., in his ' Osteographie ') to that of V a n Beneden's specimen. Its examination showed how much the premaxillaries, and especially the maxillaries and the vomer iu its basal region, had grown in massive-ness, and with this growth the form of the vomerine canal had become narrowed. I have unfortunately seen only one specimen of Mesoplodon (a specimen in the Otago Museum, Aa) in which the very beginnings of the change are present. In this young specimen there was an elevation in the mid-line of the bottom of the groove, but the vomerine trough was otherwise in no way different in shape and smoothness from that of the young forms already described. The form and thickness of the premaxillaries and their general contour were characteristic of the undeformed ziphioid snout. In a specimen of a very young cranium of Ziphius cavirostris (Epnodon chathamensis, Hector), which I was fortunate enough, during m y visit to the Chatham Islands, to examine, changes had occurred in the rostrum very similar to those which take place in the genus Mesoplodon, but of a more pronounced character. If we follow the changes in Ziphius they will, I think, help to explain those that occur in Mesoplodon. The section (Plate XIV. fig. 1) taken from the specimen in the Canterbury Museum, "a very old female" according to Von Haast (Tr. N . Z. I. vol. ix. p. 430), will show |