THE SKELETON OF THE FROG. THE SKULL. 157 



posterior end of the cranium just in front of the exoccipitals. 

 They are largely cartilaginous, but include in their anterior 



A B 



FlG. 25. A DOKSAL AND B VENTRAL VIEW OF THE CRANIUM OF A COMMON 



FROG (Eana temporaria) from which the membrane bones have 

 mostly been removed. x2 (after PARKER). 



1. sphenethmoid. 



2. palatine. 



3. pterygoid. 



4. quadrate. 



5. columella. 

 6 exoccipital. 



7. ventral cartilaginous wall of 

 cranium. 



8. 



pro-otic. 

 9. anterior fontanelle. 



10. right posterior fontanelle. 



11. quadratojugal. 



12. nasal capsule. 



II. V. VI. IX. X. foramina for 

 exit of cranial nerves. 



walls a pair of irregular cartilage bones, the pro-otics (figs. 

 24 and 25, 8). The cartilaginous area lying ventral to the 

 pro-otic and external to the exoccipital is pierced by a rather 

 prominent hole, the fenestra ovalis, which forms a com- 

 munication between the internal ear cavity, and a space the 

 tympanic cavity, which lies at the side of the head, and is 

 bounded externally by the tympanic membrane. The fenestra 

 ovalis is occupied by a minute cartilaginous structure, the 

 stapes, and articulated partly to this and partly to a slight 

 recess in the pro-otic is the columella (fig. 25, B, 5), a rod in 

 part bony and in part cartilaginous, whose outer end is 



