EHINOCEEOTID^. 



299 



the front and otkers 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 B. unicornis 

 is smaller than the same tooth in B. javanicus, and appears to be 

 earlier shed ; for in two of the skulls it has entirely disappeared iwith 

 the alyeolus 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 the comparative size of the teeth 

 with regard to each other in the series. 



As to presence or absence the small central lower incisor teeth seem 

 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 H. javanicus there are no central lower in- 

 cisor 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 varies in the different species, and is very 

 characteristic. In B,. javanicus and i2. nasalis it is large, roundish, 

 nearly £is wide as high. In M. unicornis and B. stenocephalus it is 

 narrow, oblong, erect, about twice as high as wide. In Ceratorhinus 

 sumatranus it is very large, rather irregular-shaped, forming a con- 

 siderable part of the cheeks of the skuU. 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 

 B. 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 

 to have belonged to a male. The size and form of the cavity is, no 

 doubt, greatly influenced by the age of the animal. The masseter 

 muscle becomes thicker and shorter as the animal increases in age, 

 the transverse width of the skull under the muscles becoming less 

 as the animal becomes more aged (see some measurements, showing 

 the fact, under R. javanicus). The same is shown to be the case in 

 the series of skulls of B. unicornis. 



