428 



MANUAL OF ELEMENTARY ZOOLOGY 



front of the basisphenoid and is connected with it by cartilage. With 

 the presphenoid are fused at the sides a pair of cartilage bones known 

 as the orbitosphenoids , which form the lower part of the lateral walls of 

 the cranium in the orbital region. Above them the frontals, a pair of 

 large, oblong membrane bones, complete the side walls and form the 

 roof, each bearing a large supraorbital ridge. (4) The front wall of the 

 cranium is formed by a partition of cartilage bone, knnwn as the cribri- 

 form plate, pierced by a number of holes, through which the olfactory 

 nerves pass to the nasal capsules. 



We have seen that the occipital and parietal rings are separated on 

 each side of the cranium by a gap, in which stand the auditory capsule 





CJ te> .^ 



B>^r*rl 



Fig. 312. — A diagram of the skull bones of a mammal (partly 

 after Flower and Weber), the membrane bones shaded. 



B.O., Basioccipital ; E.O., exoccipital ; C, condyle; .S". O. , supraoccipital ; 

 Par., parietal; Fr., frontal; Na., nasal; Pnu:., premaxilla; M.E., 

 mesethmoid £ L., lachrymal ; Tu., turbinal ; P.S., presphenoid ; O.S., 

 orbitosphenoid ; A.S., alisphenoid ; B.S. , basisphenoid ; SQ., squamosal ; 

 P., periotic; T., tympanic; PL, palatine; Pt. t pterygoid; Mx., 

 maxilla ; /»., jugal ; T.H., tympanohyal ; S.H., stylobyal ; E.H., 

 epihyal ; C.H., ceratohyal ; B.H., basihyal ; Th.H., thyrohyal ; 

 vomer ; MN., mandible. 



and squamosal. The latter is a large membrane bone which abuts on 

 the parietal, frontal, alisphenoid, and orbitosphenoid. From its outer 

 surface there arises a stout zygomatic process , which bears on its under 

 side the fossa for the articulation of the lower jaw and, beyond the facet 

 bends downwards to join another bone, the jugal, presently to be 

 mentioned, thus forming the zygomatic arch or cheek-bone (see p. 389). 

 From the hinder border of the squamosal a slender post-tympanic process 

 extends backwards. The auditory capsule consists of a large cartilage 

 bone known as the periotic, which ossifies in development from three 

 centres, one of which represents the prootic. This bone fits loosely 

 into a gap between the squamosal and the exoccipital. Its inner part 

 is, dense and known as the petrous portion ; this encloses the auditory 

 labyrinth. The outer part, which shows on the surface of the skull, 



