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Show 224 MR. H. O. FORBES ON T H E [Feb. 28, manner; it takes place inside the cartilage, and must, if it were coalesce with the vomer and premaxillaries, grow downwards. In Clymenia the ossification in the ethmoidal cartilage takes place in the same way, from above downwards, so that it is apparent that in Mesoplodon the ossification of the rostral elements proceeds differently. They may obtain material from the mesorostral cartilage for a time, but at all events when the vomerine element has extended above the level of the premaxillaries the cartilage must have become too attenuated to be able to provide any longer the material necessary for such a mass of bone as is developed in the British Museum specimen of Z. cavirostris, in which the resulting mesorostral bone is far greater in all dimensions than the original cartilage. Sir W . Flower remarksl that this " ossification has not hitherto been found wanting in any thoroughly adult example of any species of Mesoplodon or Ziphius; on the other hand, it appears never to occur either in Hyperoodon or Berardius." This I have found to be true, for in the Berardius arnuxii which I brought from the Chatham Islands there is an unusually long mesorostral ossification extending to nearly three fourths of the length of the snout; but it is not an ossification 2 of the same character as that in Mesoplodon, though Mesoplodon and Berardius have such close affinities. The fossilization of such a specimen of Berardius as this might perhaps result in a form like Choneziphius 3, in which the ossification has apparently proceeded, as in Berardius, from above downwards. To return to Mesoplodon grayi, it will be seen from the sections (Plate X V . figs. 1, 2, 4, v) that the premaxillaries, by growing in upon the keel of the vomer, have induced a considerable thickening in that region. It is not improbable that this pressure is the cause of the proliferation of the osseous tissue in other parts of the vomer. In some cases the maxillary ingrowth also in this region actually cuts the bone into two parts, leaving its lower portion,-that emerging as a bar on the palatal surface between the pterygoids,-as a loose fragment kept in place by the maxillaries (Plate X V . fig. 5, rnx.i &v). In the section it may be seen how shallow and compressed the outline of the original trough has become; that the sides of the premaxillaries are no longer horizontal, but perpendicular. The fragment (meth) seen in Plate X V . fig. 5, fitting into a depression in the vomerine groove, is exceedingly interesting. It represents the ossified anterior prolongation of the mesethmoid. It is complete in its anterior termination, it has never extended 1 L. c p. 419. 2 It is an ossification of the ethmoidal cartilage. 3 In describing Choneziphius packardi (Quart. Journ. Geol. Soc. xxvi. (1870) p. 503), Prof. Ray Lankester says :-" Below and posteriorly to this most anterior part of the rostrum is a cavity f of an inch in diameter, extending axially to the rostrum (pi. xxxiii. figs. 1 & 3, v. c), the remains of the primitive troughlike cavity of the vomer, as Prof. Huxley calls it in describing Belemnoziphius'' This appears to m e to imply that the ossification had been proceeding from above downwards at the time of the death of C. packardi. |
