142 Mammalia. Organ of Hearing. 



in many directions. Diving animals have valves*, or an- 

 nular sphincter musclesf, whereby the nasal passages may be 

 closed internally. 



Special. 



Olfactory Organ absent. 



All Cete possess the * crura rhinencephali '; but the baleen-bearing 

 "Whales alone of this Order have olfactory nerves ; the family Del- 

 phinidce presenting the exceptional instance of the absence of the organ 

 of smell. 



Cribriform plate. 



Ornithorhyncus forms the single exception in the Mammalian class 

 to the constant presence of a cribriform plate : in this species one 

 olfactory nerve quits each rhinencephalon and escapes from the skull 

 by a single foramen ; not dividing until it enters the nasal cavity. 



Accessory cavities : Turbinated bones. 



The Cete have no accessory cavities in the cranial bones in con- 

 nexion with the nasal cavity. In the Dolphin and Narwhal there 

 are no turbinated bones. 



D. Organ of Heading. 



The Organ of Hearing is commonly formed throughout 

 the Mammalian Class on the same general plan that it is in 

 Man : the chief distinctive features of the Class being 



1. The extension of the Cochlea into coils, suggestive of the 

 name : 



2. The ossification of the Cartilages between the stapes 

 and tympanum, forming the * malleus ', and commonly also 

 the * incus': and 



3. The presence, save in most swimmers and burro wers, 

 of an external ear or conch. 



It is only in the Orangs and Chimpanzees that the parts 

 defined in the human auricle are represented : the Concha^ 

 when present, may be quite rudimental, and is sometimes 

 represented only by a small scale-like fold of thin integu- 

 ment, contrasting very markedly with the trumpet- shaped 



* Ornithorhyncus. f Seal. 



