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Show 1893.] CETACEAN GENUS MESOPLODON. 225 further, and moreover it is not ossified to the other bones, a matter of fact, it remains for a long time separate. It may become one with the mesorostral bone in very aged animals; even then the suture remains generally very distinct, wedged in between the upgrowths of the mesorostral. A section of the same snout (Plate XV. fig. 3) taken more anteriorly is also of great interest, for there the thickening and ingrowth of the premaxillary bones are seen to bisect the vomer into two parts just below the spout; the growth of premaxillary ossifications (pmx.o) on both sides has compressed and folded together in the middle the vomerine walls, thickened already by proliferation of their tissue, the point of union being with some care observable in the median line. In a still more anterior section (Plate XV. fig. 4, v) the keel portion of the vomer below the bisection has increased in growth and appears as a round rod, part of which shows on the palatal surface, and has begun to become implicated in the ivory-like ossification which has commenced. In the Chatham Island specimen and in that in the Otago Museum (G and F respectively in above list) much the same changes occur. In some cases, as, for instance, in the female specimen (H in the above list) in the Canterbury Museum, the filling-up of the vomerine spout has proceeded more symmetrically, and we have then greater regularity in the form of the section of the snout (Plate XIV. fig. 4, a). Fig. 1 b, Plate XII., represents a section through the middle of the snout of the type (male) specimen of M. grayi in the Canterbury Museum, and how widely it differs from that of the female of the same species in the same Museum (Plate XIV. fig. 4 a) or of M. australis in the British Museum (Plate XIII. fig. 2) is at once apparent-yet not greater than the difference between the three forms of Mesoplodon layardi shown in tbe sections a, b, c, fig. 2, p. 228. Fig. 2, Plate XII., is reproduction of the section of M. haasti from Sir W . Flower's paper in the Trans. Zool. Soc, so often referred to, and which, as he has pointed out, differs so much from the section of any other he has examined that he could not include it in any known species ; while fig. la, Plate XII., is a section made by me of the type specimen of M. grayi. in the Canterbury Museum, somewhat more anterior than fig. 1 b, but still in the region where the vomer appears on the palatal surface, and their similarity will be at once admitted. I have already quoted Sir W . Flower's remark that if so great a change can take place, due to individual variation, as exists between M. grayi and M. haasti, then most of the fossil species based solely on the form of the rostrum are quite valueless. If we take, for instance, the two forms M. angulqtus and M. medilineatus, there exists far less difference between them than between some of the forms of M. grayi or of M. layardi. The median lines or sutures on the surface of the mesorostral bone, which vary so much, and also the gibbosities of the premaxillaries, are, after studying the sections of immature forms, PROC. ZOOL. Soc-1893, No. X V . 15 |