XIIL] THE FROG. 217 



with the corresponding concave facet on the 

 front of the atlas. 



c. The bone bearing the condyle on each side 

 and, with its fellow, enclosing the foramen mag- 

 num, is the exoccipital. 



d. The thick bone running outwards in front of 

 the exoccipital, on each side, protects the front 

 part of the internal ear, and is the pro-otic bone. 



e. Between these two bones, on the outer side of 

 the chamber which contains the organ of hear- 

 ing (periotic capsule), is a cartilaginous inter- 

 space containing an oval aperture, ihefenestra 

 ovalis. In this is fixed the inner end of a partly 

 cartilaginous and partly osseous rod, the colu- 

 mella auris. 



/. Attached to the outer end of the pro-otic bone 

 is a hammer-shaped bone the sguamosal, which 

 extends from the pro-otic bone to the articu- 

 lation of the lower jaw. 



2. The roof of the skull. 



a. Passing forwards from the exoccipitals are two 

 long flat bones, the par ieto-f rentals, one on each 

 side of a median suture which answers to the 

 sagittal and frontal sutures in man. 



b. In front of these come two triangular bones 

 the nasals. 



c. In front of the nasals are two other bones, which 

 belong rather to the ventral than to the dorsal 

 face of the skull. They form the extreme front 

 of the snout, and each sends a process towards 

 the nasals; these are \hQpremaxillary bones. 



