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Show 1867.J DR. J. E. GRAY ON THE RHINOCEROTIDAE. 1007 I at first had a difficulty in distinguishing the difference between tbe skulls of the Javan and Sumatran species ; but this arose from the British Museum having received from the Leyden Museum, through M . Franks, a skeleton of the Javan species under the name of R. sumatranus. But when I received a skull of the two-horned species from Pegu, the mistake in the name of the skeleton was soon discovered. Some of tbe specimens of skulls of R. unicornis and R. javanicus in the British Museum have the foramen in the front of the eye over the front and others over the hinder edge of the second premolar. In both the specimens of R. sumatranus it is over the back edge of the first premolar. The first premolar in the three adult specimens of R. unicornis is smaller than the same tooth in R. javanicus, and appears to be earlier shed ; for in two of the skulls it has entirely disappeared with the alveolus that contained it, and in the other one the tooth is there, but it is nearly rootless and the alveolus is nearly absorbed. The two large lateral lower cutting-teeth have a sharply keeled inner edge ; but the teeth often wear almost entirely away, so that this form is lost. The grinders of the milk or first series have much larger and more equal folds on the outer side than those of the permanent set;. in the latter the front fold is linear and near the front margin of the tooth. The teeth in some specimens appear to be rather smaller than in others, but there is a difference in tbe comparative size of the teeth with regard to each other in the series. The presence or absence of small central lower incisor-teeth seems to be liable to considerable variation. In one adult skull from India there are two incisor-teeth ; and in another there are two holes, but they are crowded together and are closing up. In three specimens of R. javanicus there are no central lower incisor teeth, nor space for them ; between the two large ones in the two other skulls, which are from younger animals, the central lower incisor teeth are well developed and cylindrical, being much the largest in the smaller and younger specimen. The lachrymal bone va'ries in the different species, and is very characteristic. In R. javanicus and R. nasalis it is large, roundish, nearly as wide as high. In R. unicornis and R. stenocephalus it is narrow, oblong, erect, about twice as high as wide, In Ceratorhinus sumatranus it is very large, rather irregular-shaped, forming a considerable part of the cheeks of the skull. It differs a little in size and form in the specimens of the same species, but retains its general and distinctive forms. There is a considerable variation in the size and form of the cavity under the zygomatic arch in the skulls that appear to belong to the same species. Thus in the four specimens of R. unicornis, which are nearly adult, two of them have the cavity short and broad, and two long and narrow. The same may be observed in the skulls of R. javanicus and R. nasalis. The aperture is widest, compared with its length, in the oldest specimens. This may probably be a sexual distinction. One of the skulls with a short wide opening is known |
