EINAR LÖNNBERG, THE GENUS POTAMOCHCERUS. 5 



273 mm. the parietal area measiires 32 mm., and in an old 

 specimen with the length of the skull 318 mm. it has incre- 

 ased to 46 mm. The other measurements, as is seen on the 

 table, are also rather similar from Liberia to Cameroons in 

 specimens of similar age. As a rule the absolute dimensions 

 increase with age, but there is also a certain transformation 

 taking place, the defensive organs being espesially enlarged in 

 cider males. The apophyses above the canines become much 

 thicker and the exostoses at their upper end as well as the 

 corresponding ones on the nasals and adjoining parts of the 

 maxillaries are much enlarged. The whole nasal region be- 

 comes broader and forms on either side a roof-like ridge 

 above the vertical portion of the maxillary. I have seen 

 an old specimen from Cameroons in which the exostoses on 

 the apophyses have joined with the upper ones from the 

 maxillary so as to form a complete tube-like canal above and 

 behind the canines, although the apophyses themselves are 

 not so high as in other species. The anterior portion af the 

 zygomatic arch formed by jugale itself is very strongly thick- 

 ened with age and this causes that the greatest zygomatic 

 width is in old specimens found just across the thickened 

 jugaha at the anterior half of the arch, while the greatest 

 width in younger, although adult specimens is found at the 

 posterior squamosal part of the arch.^ The low^er jaw as 

 well is thickened with age and at the same time its angular 

 portion becomes much broader, which makes an old skull look 

 higher at the side of a younger. These changes with age and 

 growth do not, however, satisfactorily explain the difference 

 in the relative breadth of the parietal area when compared 

 with the upper mesial length of the skull. The percentage 

 expressing this relation is in the boar from Liberia 11,5 ^/o 

 in the specimens from 



the Niger-Expedition resp. 12,: and 11,6% 



in the specimen from Gaboon 11,5 



' By this increase in thickness of the jugalia with age another charac- 

 teristic of the P. porcus- group becomes more pronoimced as well. This 

 the shape of the anterior wall of the orbit together with the base of the 

 zygomatic arch which have a more vertical anterior surface in the P. por- 

 \cus group than in the P. choeropotamus group in which it is more obli- 

 quely sloping. In the former the anterior part of the zygomatic arch forms 

 an angle against the longitudinal axis of the skull which is 90° or nearly 

 so, especially in adult boars, while in the latter group this angle is much 

 larger than 90°. This is most easily observed, if the skulls are seen from above 



