232 ANATOMY OF VERTEBRATES. 



in number -jf'-ri^O '> a ^ fixed in deep and distinct sockets, sepa- 

 rated by interspaces which admit of the close interlocking of the 

 upper and lower teeth when the mouth is closed ; the longest and 

 largest teeth are at the middle of the series, and they gradually 

 decrease in size as they approach the ends, especially the pos- 

 terior one. 



In the common Dolphin the number of teeth amount to 190, 

 arranged in equal numbers above and below, and there is a pair 

 of teeth in the premaxillaries which are toothless in the other 

 Cetacea. They have slender, sharp, conical, slightly incurved 

 crowns, and diminish in size to the two extremes of the dental 

 series ; the acute apices are longer preserved than in the foregoing 

 species. 



The Gangetic Dolphin (Platanista gangetica) differs from the 

 rest of the Delphinida scarcely less in the form of its teeth than 

 in that of the jaws. Both the upper and lower maxillary bones 

 are much elongated and compressed ; the symphysis of the lower 

 jaw is coextensive with the long dental series, and the teeth rise 

 so close to it that those of one side touch the others by their 

 bases, except at the posterior part of the jaw. The lateral series 

 of teeth are similarly approximated in the upper jaw at the median 

 line of union, which line is compelled, by the alternate position 

 of the teeth, to take a wavy course. There are thirty teeth on 

 each side of the upper jaw, and thirty-two on each side of the 

 lower jaw. In the young animal they are all slender, com- 

 pressed, straight, and sharp-pointed, the anterior being longer 

 than the posterior ones, and recurved. Contrary to the rule 

 in ordinary Dolphins, the anterior teeth retain their prehensile 

 structure, while the posterior ones soon have their summits 

 worn down to their broad bases : in the progress of their growth 

 the implanted base is elongated antero-posteriorly, its outer 

 surface augmented by longitudinal folds analogous to those in 

 the teeth of the Sauroid fishes. Sometimes the posterior tooth 

 of Platanista has the base divided into two short fangs, the 

 sole example of such a structure which I have met with in the 

 existing carnivorous Cetacea. In the Dolphins of the South 

 American rivers (Inia) the inner side of the tooth expands into a 

 crushing tubercle. 



The primitive seat of the development of the tooth-matrix is 

 maintained longer in the Cetacea than in other Mammalia ; a 

 greater portion of the tooth is also developed before the matrix 

 sinks into, or is surrounded by, a bony alveolus ; and, with the 

 exception of the rudimeiital tusks in the Narwhal, is at no period 



