82 Mr. G. R. Waterhouse on the Rodentia. 



being extended backwards behind the last molars ; they are 

 destitute of longitudinal grooves. The molars are f^f- ; the 

 entering folds of enamel are very simple, there being but one 

 external and one internal deep fold to each tooth. 



In Orycterus the upper incisors are shorter and much more 

 curved, and are furnished with a deep longitudinal groove in 

 front ; those of the lower jaw are destitute of grooves, are very 

 deep from front to back, and slightly concave at the sides. 

 The molars are |^, nearly cylindrical, but their anteropos- 

 terior diameter is the shortest, excepting in the front molar 

 of the lower jaw. In the skulls before me the teeth are worn, 

 and exhibit scarcely any trace of folds of enamel entering 

 into the body of the tooth. 



In both genera the enamel of the incisors is colourless, and 

 the incisors of the lower jaw are extended backwards to the 

 condyle. 



The skull of the Bathyergidce is rather broad, much con- 

 tracted between the orbits, and (as in other Rodents which 

 live for the most part underground*) the cranial cavity is pro- 

 portionally small. The nasal bones are narrow and elongated. 

 In Orycterus they are nearly of equal width throughout, 

 whilst in Bathyergus capensis they are considerably broader 

 behind than before. The anterior root of the zygomatic arch 

 is formed entirely of the superior maxillary bone, and incloses 

 a small ant-orbital opening : the lower boundary of the zygo- 

 matic process of the maxillary is slightly concave, so that the 

 anterior portion of the zygomatic arch is slightly thrown up 

 from the plane of the palate. This is more decidedly the case 

 in Bathyergus Capensis, in which the ant-orbital foramen is 

 rather larger. The malar bone is of moderate size, and is ex- 

 tended backwards so as to enter into the composition of the 

 glenoid cavity. The portion of the palate situated between 

 the molar teeth is remarkably contracted, and this part de- 

 scends below the level of the anterior portion of the palate, 

 and is continued some distance beyond the last molar. The 

 incisive foramina are very small, and there are no openings in 

 the posterior portion of the palate. The glenoid cavity of the 

 temporal bone is very broad. In Orycterus the posterior por- 

 tion of the malar bone forms a longitudinal ridge, which no 

 doubt tends to restrain to a certain extent the lateral motion 

 of the condyles of the lower jaw, but in B. capensis this ridge 

 is wanting. The auditory bullae are of moderate size — larger 

 in Orycterus maritimus than in B. capensis. In the former 

 animal the upper surface of the skull forms nearly a straight 



* Compare the skull of the burrowing Marmots with that of their con- 

 geners the Squirrels, and that of Spalax with the Rats. 



