222 MJB. H. o. FORBES OS 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 

 lettereclAabove(seefig.l,p.221),sectionsof the rostrum taken at 11, 

 64, 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 diver- 

 ticulum of the maxillary (as seen in Clymenia, and more markedly in 

 Fhi/seter) 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 h). On the outside the premaxillaries articulate with the 

 maxillaries. Still more anteriorly (Plate XIV. fig. 2 a) there is a 

 slight change in the form of the vomerine keel, and the premaxil- 

 laries 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, bet\\een the ^^■ings of the 

 vomer. 



The next older specimen (B) I take to be of greater age than that 

 of the skeleton in the Eoyal College of Surgeons, figured by Sir 

 William Plower, and about equal (judging alone by the figure, plate 

 Ixii., in his ' Osteographie ') to that of Van Beneden's specimen. Its 

 examination showed how much the premaxillaries, and especially the 

 maxillaries and the vomer in its basal region, had grown in massive- 

 ness,aud with this growth the form of the vomerine canal had become 

 narrowed. I have unfortunately seen only one specimen of 31eso- 

 plodon (a specimen in the Otago Museum, An) in which the very 

 beginnings of the change are present. In this j^oung 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 uudeformed zipbioid 

 snout. In a specimen of a very young cranium of ZipMns caviros- 

 tris (Ejnodon chathamensis, Hector), which I was fortunate enough, 

 during my visit to the Chatham Islands, to examine, changes bad 

 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 Zipliius 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. X. Z. I. vol. ix. p. 430), will show 



