352 CETACEANS. 



slender, sharp, conical, slightly incurved crowns, and diminish 

 in size to the two extremes of each dental series ; the acute apices 

 are longer preserved than in the foregoing species. In PI. 88, fig. 2, 

 the teeth of the upper jaw of an immature Dolphin are exposed 

 in situ, and both the extent of their implanted base and that of 

 their pulp-cavity is displayed. 



The teeth of the common Porpoise fPhoccena vulgarisj, are 

 arranged in equal number on each side of both upper and lower 

 jaws, and are from 80 to 92 in number ; the crown is slightly 

 expanded and compressed, and the fully-formed fang is recurved 

 and enlarged at its extremity. 



The gangetic Dolphin fPlatanista gangeticaj differs from the 

 rest of the BelphinidcR 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 co-extensive 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(l) ; in the young animal they are all 

 slender, compressed, 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. The most remarkable change that 

 occurs in the progress of growth is the antero-posterior expansion 

 as well as elongation of the implanted base of the tooth, which 

 likewise has its outer surface augmented by longitudinal folds 

 or indentations, analogous to, but weaker than those in the base 

 of the teeth of Sauroid fishes. Sometimes the posterior tooth of the 

 Platanista has the base divided into two short fangs, — the sole 

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



(2) PL 90, fig. 7. 



