1893.] CETACEAN GENUS MESOPLODON. 235 



In the youngest Otago speciiuea the tympanic bone was 1| inch 

 in length and 1 inch in breadth at its posterior end, and the older 

 1^^ inch in length by Ig in width, where it is divided by a deep 

 groove, as in the species of M. grayi described by Sir W. Plower 

 and Sir J. von Haast. Except for a slight difference in size these 

 bones are almost indistinguishable in the different specimens in 

 which they are present. 



Mandible. 



The table of measurements, p. 231, gives the data by which the 

 mandible of the specimen A (the young Otago specimen) may be 

 compared with that in the Museum of the Eoyal College of Surgeons 

 specimen and with Yan Beneden's iigure — all immature ; with that 

 of r, the older example in the Otago Museum, H, the Canterbury 

 Museum female, and the type K and other fully adult specimens. 

 TLie teeth in the mandible of A are half opposite, half behind the 

 postei'ior end of the symphysis. In the specimen H, the centre of 

 the teeth is 2*1 inches anterior to the hinder end of the symphysis ; 

 in F it is 10*4 inches from the tip of the mandible, and '80 inch in 

 front of the hinder end of the symphysis, while their posterior 

 margins are well anterior to the same point. The teeth are erect, 

 equiangular, and slope outward, with the apex slightly incurved. 

 The socket is large enough to allow of a slight play of the tooth in it. 

 The dentary groove bulges out on both sides opposite the tooth 

 from 5-V— 0^ inch. In the type K the centres of the teeth are 

 opposite to the posterior end of the symphysis. 



Summanj. 



The above observations ha^•e, I think, shown that in the genus 

 Mesojjhdon the mesorostral bone is formed, not at all events by the 

 sole and direct ossification of the mesorostral cartilage, but in 

 great part by a proliferation of the osseous tissue in the floor and 

 sides of the vomer, and in the walls of the premaxillaries, caused 

 probably by the compression of these bones, as a result of the 

 vigorous growth that seems to arise at an early age in the maxillary 

 and premaxillary bones surrounding them, and originated perhaps 

 also by the movement upon each other of the maxillaries and pre- 

 maxillaries ; that the form assumed by the rostrum when viewed 

 in section varies very greatly with the age and sex of the indi\idual ; 

 and that the outline of a transverse section of the rostrum can no 

 longer be considered as a character for separating the species of 

 the genus. It becomes necessary also to unite, ss I have done in 

 this jjaper, the forms described under the names of Ifesoplodon 

 australis, Flower, M. Juiasti, I'lower, M. Juriori. Gray (of Hector, 

 but not of Flower), under the same species M. (jniyi, Haast. It 

 follows also that a great number of the Crag fossils of the genus 

 Mexophdon must be united together as forms of one species, of 

 different sexes and ages. 



