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Show 1893.] CETACEAN G E N U S M E S O P L O D O N . 235 In the youngest Otago specimen the tympanic bone was 1| inch in length and 1 inch in breadth at its posterior end, and the older l-f-2- inch in length by lg in width, where it is divided by a deep groove, as in the species of M. grayi described by Sir W . Flower 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 Royal College of Surgeons specimen and with Van Beneden's figure-all immature ; with that of F, the older example in the Otago Museum, H , the Canterbury Museum female, and the type K aud other fully adult specimens. The teeth in the mandible of A are half opposite, half behind the posterior end of the symphysis. In the specimen H , the centre of the teeth is 2 T 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 Jg-•il inch. In the type K the centres of the teeth are opposite to the posterior end of the symphysis. Summary. The above observations have, I think, shown that in the genus Mesoplodon 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 premaxillaries ; that the form assumed by the rostrum when viewed in section varies very greatly with the age and sex of the individual; 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 paper, the forms described under the names of Mesoplodon australis, Flower, M. haasti, Flower, M. hectori, Cray (of Hector, but not of Flower), under the same species M. grayi, Haast. It follows also that a great number of the Crag fossils of the genus Mesoplodon must be united together as forms of one species, of different sexes and ages. |
