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Show 1893.] CETACEAN GENUS MESOPLODON. 223 more clearly than words that there has taken place a very great increase in the maxillaries and in the premaxillaries, which latter also (as in the young M. grayi just alluded to) come down and appear on the palatal surface of the rostrum, intervening between the vomer aud the maxillaries. The vomer, it will be seen, has lost all its usual form by being squeezed ; the trough is only indicated by a small depression between its two thickened arms (Plate XIV. fig. 1, v.tr). In the British Museum specimen of the same species, of which there is a complete skeleton, the vomer is in its upper aspect a sharp, triangular, ridged bar, very like that in Mesoplodon angulatus, one of the fossil forms from the Red Crag. There is an enormous thickening of the premaxillaries, as well as of the vomer. In the older specimens in which the great prenasal fossa is seen, the vomer forms the bottom of the fossa and the mesethmoid disappears. Into how extraordinary a feature this eventually grows up in the aged individual is well illustrated by the two crania in the British Museum collection. The species originally described by Sir James Hector as Epiodon chathamensis has now been united by Sir W . Turner to Z. cavirostris, a determination acquiesced in by Hector. The differences exhibited by the Chatham Island specimen and the other two crania in the British Museum are so very marked that it appears difficult for m e to conceive how one form can ever grow into the other. If the identification be correct, and I have no reason to question it, it will be found that only in the one sex- probably the male-does this enormous development of the vomer take place, accompanied or preceded by the formation of the great prenasal cavity, irom which the species derives its name. In N e w Zealand both forms occur ; and I have examined specimens in which this prenasal cavity was already deep, but which were younger (cf. Tr. N . Z. I. vol. v. pi. iv.) than either of the specimens in the British or the Canterbury Museums, as indicated by the less advanced stage of the vomerine upgrowth. Sir W . Turner has remarked on the abrupt manner in which the posterior end of the mesorostral bone terminates and on the smoothness of the cavity. This is observable in all the N e w Zealand forms, and the appearance suggests that, through some cause or other, absorption takes place, or disease attacks these bones in the male and not in the female. It is to be noted that in many of the specimens of this species the accretion of material and the change of form are confined to the vomer, as is seen in the Canterbury Museum specimen and in that figured by Van Beneden-at least for some considerable time there is no deposit of osseous tissue in the premaxillary portion of the spout, to which the cartilage also extends. If the filling-up of this trough were the result of ossification alone of the rostral cartilage, it would proceed, it seems to me, uniformly over the wmole surface of the trough. If we examine also, in connection with this, the anterior prolongation of the ethmoid as it occurs in Berardius arnuxii, it may be observed that the ossification proceeds in quite a different |