376 



A]S ATOMY OF VERTEBKATES. 



242 



Skull of the Spalax typhlus. 



almost equal develo2:)ement of the coronoid and angular processes, 

 on each side the base of the narrow process supporting the 

 condyle. 



In the Great Mole-Kat ( Oryctei^ojms capensis), the occipital 

 region of the skull is very broad and low. The compressed 

 paroccipitals project downward and backward. The auditory 

 bulla is pyriform, its apex articulating with the pterygoids. The 



temporal fossae meet along 

 a well-developed crista ex- 

 tending from the interor- 

 bital region to the strong 

 transverse superoccipital 

 crest. The squamosal forms 

 a horizontal plate, with a 

 curved border extending 

 from the root of the zvo;oma 

 to above the ' meatus ex- 

 ternus,' which is directed 

 upward and forward. The 

 zygomatic arches are strongly carved outward. The premaxil- 

 laries extend further backward than the nasals : these are very 

 long and narrow. In the Blind Mole-Rat ( Spalax typhlus), the 

 orbit, fig. 242, o, is not defined : the great antorbital vacuity, v, 

 might be mistaken for it. 



In the skull of the Cape Jerboa {Helamys capensis), the occi- 

 pital region, owing to the enormous developement of the acoustic 



bullae, appears as a broad shallow 

 depression between them at the 

 back part of the skull. The par- 

 occipitals are small, slender, sub- 

 elongate, and project downward, 

 distinct from the bullae. The 

 broader mastoid processes are ap- 

 plied to the outer side of the 

 petrosal portion of the bullae : the 

 swollen bases of the mastoids form a tract upon the upper surface 

 of the cranium larger than the interparietal bone, on each side of 

 which they are situated. The slender posterior clamping processes 

 of the squamosals impress the outer sides of the bullae which they 

 support, above the ' meatus externus : ' this canal is directed 

 upward and a little outward. The parietals are pushed by the 

 squamosals entirely to the upper region of the cranium : the 

 sagittal suture remains, as well as the frontal one. The temporal 



243 



of the Jerboa (Dipus Sagitta). 



