192 VERTEBRATE SKELETON 



seals, whales and man (the foramina themselves closed in whales) in all of which 

 Jacobson's organ is rudimentary. When present, the organ is supported or more 

 or less enclosed by the paraseptal cartilage which is connected in front with the 

 septum, behind only in young marsupials. 



The nasal cavities may be increased by the formation of cavities 

 (sinuses) in the adjacent bones (frontal, maxilla, ethmoid, sphenoid) 

 which are Hned by extensions of the olfactory epithelium, their 

 excavation beginning at about the time of ossification. The frontal 

 sinuses are especially developed in elephants, extending through 

 parietals and squamosals nearly to the occipital condyles. The 

 sinuses are large in Ruminants, Perissodactyls and some Marsupials, 

 but are lacking in some Primates, many rodents and Edentates. 



The otic capsules, relatively smaller than in other groups, are 

 largely in the cranial floor because of the large brain. Each has from 

 four to six ossification centres, a larger number than is known in 

 other Amniotes and possibly including some of the extra ones of 

 fishes, or two or more centres of mammals may represent one in the 

 lower groups. 



One centre is in the dorso-lateral part of the anterior cupula, a second over 

 the inner tip of the cochlea, and a third on the outer posterior end of the capsule, 

 these three usually being regarded as the homologue of the prootic. The 

 epiotic centre is higher on the postero-lateral end, the opisthotic on the ventral 

 side between the fenestra and a sixth centre, more caudal on the medial side. 

 These homologies have not been proved. 



These separate centres fuse early to a single petrosal (periotic) 

 bone, articulating with the basioccipital, basisphenoid, exoccipital 

 and alisphenoid. This petrosal bone is firm and hard (whence the 

 name), encloses the labyrinth and has vestibular and cochlear 

 fenestrae in its lateral wall. The foramen lacerum is between it 

 and the alisphenoid, the jugular foramen behind it. These openings 

 and sutures render the petrosal somewhat less firmly attached to 

 the rest of the cranium, and in whales the connexion is so slight that 

 it is readily separated from the rest, forming 'cetoliths.' The 

 mastoid, a separate cartilage bone (sometimes absent) Hes posterior 

 to the petrosal with which it usually unites, but it never con- 

 tains any part of the labyrinth. It sometimes has a paramastoid 

 process on its lower surface; but when fused with the petrosal, the 

 process is called the mastoid process. Its position at the postero- 

 lateral angle of the skull suggests comparison with the parotic or 



