THE PHOCODONTIA. 349 



tliey are long. In the upper view they leave not more than a 

 seventh of the length of the cerebellum exposed, while they 

 overlap it largely at the sides. The outer surface of the hemi- 

 spheres is extremely convoluted, the gyri being numerous and 

 separated by deep sulci. There is a well-mai-ked Sylvian 

 fissure, with a central lobe, or insula. A rudiment of a pos- 

 terior cornu has been observed, in the lateral ventricle. The 

 corpus callosum is small, relatively to the size of the hemi- 

 spheres, and the anterior commissure is almost obsolete. The 

 medulla oblongata has corpora trapezoidea. The olfactory 

 nerves are wanting — a circumstance which agrees 'with the 

 entire absence of ethmoidal turbinals. The eye has a thick 

 sclerotic, and there is a choanoid muscle ; no nictitating mem- 

 brane is present. 



The external auditory aperture is so small as to be easily 

 overlooked. The meatus auditorius is a narrow undulatinc: 

 tube about two inches long. The tympanic membrane is con- 

 cave externally ; and, as is usual in the Cetacea, is connected 

 by a ligament with the handle of the malleus. There is only 

 a small aperture in the stajDes. The tensor tympani arises, as 

 in Carnivores, from a fossa in the periotic ossification. 



The Eustachian tube passes through the notch in the ptery- 

 goid and opens into the nasal passage on the inner side of that 

 notch. Close to its commencement it communicates, by an 

 oval aperture, with a remarkable air-chamber, which extendis 

 backward between the periotic mass and the basis cranii^ and 

 forward to the under side of the expanded part of the maxilla, 

 where it opens into the canal between the maxilla and the 

 frontal already described. These chambers, like the bronchi, 

 are generally full of nematoid worms. The testes and penis 

 of the male are enormous in proportion to the size of the 

 body. 



The penis is devoid of a bone, and, ordinarily, is bent up 

 in the long preputial sheath. 



c. The JPhocodontla are represented only by Zeuglodon^ 

 Squalodon^ and other large extinct cetaceans of the tertiary 

 epoch. These remarkable fossil forms constitute connecting 

 links between the Cetacea and the aquatic Carnivora. The 

 cervical vertebrae are distinct and unanchylosed, nearly re- 

 seinbling those of the Hhyncocetl. The caudal vertebrae have 

 their transverse processes perforated vertically, as in many 

 Qetacea. The distal ends of the ribs are enlarged somewhat 

 as in the Slrenla. The skull is symmetrical, and the nasaJ 

 bones, though still short, are longer than those of any other 



