1893.] CETACEAX GEXIJS MESOPLODO??-. 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 and 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 Bi-itish 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 Afesoj^hdou angulatus, one of the fossil forms 

 froui 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 seeu, the vomer forms the 

 bottom of the fossa and the mesethmoid disappears. Into how 

 extraordinary a feature this eventually grows up in the aged indi- 

 vidual is well illustrated by the two crania ia the British Museum 

 collection. The species originally described by Sir James Hector 

 as Epiodon chatliamensis 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 me 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, from which the species derives its name. In 

 New 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 'New Zealand forms, and 

 the appearance suggests that, through some cause or other, absorp- 

 tion takes place, or disease attacks these bones iu the male and 

 not in the female. 



It is to be noted that in many of tlie 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 whole surface of the 

 trough. If we examine also, in coiniection with this, the anterior 

 prolongation of the ethmoid as it occurs in Bc.rardtus anniu^li, 

 it may be observ(,'d that the ossification proceeds in quite a different 



