342 THE ANATOMY OF VERTEBRATED ANIMALS. 



tained in the CepluiJopodn on -svhicli the Cetacean feeds. In 

 the other group of the Physeteridir, — the Ziphiinm or lihyn- 

 choceti — to which the Bottlenosed Whale {Hyperoodon) be- 

 longs, there are only one or two pairs of fullj'-formed teeth in 

 the mandible. Some recent and many fossil (middle and later 

 tertiary) genera of the Cetaceans are remarkable for the elon- 

 L^ated rostrum formed by the solid ossification and anchj-losis 

 of the ethmoid, premaxilte, and maxillse. 



The Platanistid m are fluviatile or estuarine Cetacea, 

 which occur in the Ganges and in the rivers of South America. 

 The cervical vertebrse are not anchylosed, and the costal car- 

 tilages are not ossified. The tubercula and capitula of the 

 ribs blend together posteriorly. The symphysis of the man- 

 dibles is extremely long and the jaws are narrow. Numerous 

 teeth with compressed fangs are found in both jaws. The 

 eyes are small, and in Platan ista they are rudimentary. 



In the Depldnidoe lastly (Dolphins, Porpoises, Gram- 

 puses), the teeth are usually numerous in both jaws, though 

 the Narwhal is an exception to this rule, as has already been 

 mentioned. 



The anterior cervical vertebrfe are generally anchj'lcsed 

 together. The posterior ribs lose their capitula and become 

 articulated only with the transverse processes of the vertebrae. 

 The costal cartilages are well ossified. The symphysis of the 

 mandible does not exceed one third of the rami in length, and 

 the frontal and maxillary bones are not especially produced 

 upward at their edges. 



As the common Porpoise (PhocCBna communis), which is 

 a member of this group, is the Cetacean which is most likely 

 to come within reach of the student, it may be useful to 

 speak at some length of its more interesting anatomical pe- 

 culiarities. 



The adult animal is usually about five feet long, and is 

 covered with a smooth integument upon which no hair is to 

 be discovered, though a few hairs are visible about the mouth 

 in the young animal. The contour of the anterior part of the 

 head is very convex, and presents, in the middle line, the 

 spiracle or blow-hole, which has the form of a crescent with 

 the points turned downward and forward. The eyes are small 

 end placed low down, close to the posterior end of the gape 

 of the mouth, which is bounded by dense and rigid lips. The 

 aperture of the ear lies about an inch and three-quarters be- 

 liind the eye, and is so minute as to be discovered with diffi- 

 culty. The genital aperture is placed a long way in front of 



