224 MB. H. O. PORBES ON THE [Feb. 28, 



manner ; it takes place inside the cartilage, and must, if it were to 

 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 Ilesojjlodon the ossification of the rostral elements pro- 

 ceeds differently. They may obtain material from the mesorostral 

 cartilage for a time, but at all events when the vomerine ele- 

 ment has extended above the level of the premaxillai'ies 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 dimen- 

 sions than the original cartilage. Sir W. Flower remarks ^ that this 

 " ossification has not hitherto been found wanting in any thoroughly 

 adult example of any species of Mesoploilon or Ziphius ; on the 

 other hand, it appears never to occur either in Hiiperoodon or 

 Berardins.'' This I have found to be true, for in the Berardius 

 arnuxil which I brought from the Chatham Islands there is an 

 unusually long mesorostral ossification extending to nearly three 

 fourths of the leugth of the snout ; but it is not an ossification ^ of 

 the same character as that in Mesoplodon, though Mesoplodon and 

 Berardius have such close affinities. The fossilizatiou of such a 

 specimen of Berard'ms as this might perhaps result in a form like 

 Choneziphius ^, 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 XV. 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 i)arts, 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 XV. fig. 5, mx.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 XV. fig. 5, fitting into a depres- 

 sion in the vomerine groove, is exceedingly interesting. It repre- 

 sents the ossified anterior prolongation of the mesethmoid. It 

 is complete in its anterior termination, it has never extended 



1 L. c. p. 419. 



^ It is an oseification of the ethmoidal cartilage. 



^ In describing Choneziphius packardi (Quart. Journ. Geol. Soc. sxri. (1870) 

 p. 603), Prof. Ray Lankester says : — " Below and posteriorly to this most anterior 

 part of the rostrum is a cavity | of an inch in diameter, extending asially to 

 the rostrum (pi. xxxiii. figs. 1 & 3, v. c), the remains of the primitive trough- 

 like cavity of the vomer, as Prof. Huxley calls it in describing Bclcmno::rphius" 

 This appears to me to imply that the ossification had been proceeding from 

 above downwards at the time of the death of C. packardi. 



