20 SPECIMENS OF CETACEA. 



inferior surface, where they are separated by a cleft, varying in width 

 and depth in different species, which is usually limited to the region 

 of the lobes, though sometimes, as in Tursiops and Monodon, it is 

 prolonged forward on the inferior surface as a median groove. The 

 outer lobe is smooth and rounded, the inner is usually roughened and 

 pointed, though in L. acutus, Tursiops, and Cephalorhynchus it is more 

 truncated. The outer surface has about its middle a low raised 

 process, which is continued into the lip-like mallear process on the 

 sharp upper border of this surface, but is not prolonged to the inferior 

 surface. 



The Periotic proper is a dense, hard bone, which is fused with the 

 tympanic in the adult. Its cerebral surface is pierced by one or tAvo 

 large canals for the auditory nerve ; its tympanic surface, next the 

 cavity, has two fenestrse, the ovalis and rotunda, to the former of 

 which the stapes sometimes remains attached, though the incvis may 

 have been lost and the malleus fused with the lip-like process of the 

 upper border of the outer surface of the tympanic. In the Baleen 

 Whales the periotic proper, which encloses the internal ear, is much 

 smaller than the tympanic, and is smooth and convex on its inferior 

 surface ; but cc)ntinuous with it a mass of cancellated bone, by which 

 it is attached to the squamosal, is sometimes regarded as a mastoid 

 process. 



In the Toothed Whales the periotic is not so strongly anchored to 

 the squamosal as in the Baleen Whales, and is more easily lost. In 

 many species it is not so long and broad as the tympanic, but 

 Hyperoodon and Orca form exceptions. A plate-like surface at the 

 posterior end of the periotic articulates with a corresponding surface 

 of the tympanic. In Hyperoodon, Mesoplodon, Ziphius, and Phocsena, 

 they are plane surfaces, and the articulation is one of apposition. 

 In Physeter and many Dolphins, as Globicephalus, Orca, Grampus, 

 they are concave and striated, and the articulation is feebly denticu- 

 lated. A rounded, bullet-shaped prominence, immediately below 

 and on the tympanic side of the internal auditory meatus, marks the 

 position of the labyrinth. 



For purposes of Classification the Cetacea are arranged in two sub- 

 orders, Mystacoceti and Odontoceti. 



The Mystacoceti (M.C.) (Whalebone Whales) consist of the families 

 Balsenidse (Right Whales) and Balsenopteridee (Rorquals). 



The Odontoceti (O.C.) (Toothed Whales) consist of the families 

 Physeteridff, (Sperm and Beaked Whales), Platanistidse (Fresh- water 

 Cetacea), and Delphinidae (Dolphins). 



