CETACEANS. 



359 



also increased(l)." A deeper insight into the phenomena of the 

 growth of teeth in the class Mammalia proves this to be no ex- 

 ceptional case, but one of beautiful and harmonious concordance 

 with the general laws of organic development. The Cetacea per- 

 manently represent that early embryonic stage when no cervical 

 constriction divides the large head from the trunk, and when the 

 rudimental limbs offer no outward marks of joints or digits ; they 

 likewise retain a preponderating proportion of brain, and manifest 

 for a long period, and on a magnified scale, the first stages in the 

 development of the teeth. 



When by the increasing depth of the jaw and the reciprocal 

 elongation of the tooth its base or fang becomes supported by bone, 

 a longer time than usual elapses before the alveolus is completed 

 by the development of transverse partitions between the outer and 

 inner walls of the open groove, and in the meanwhile the teeth are 

 lodged like those of the Ichthyosauri in a common and continuous 

 bony channel. In the Delphinida the teeth are successively developed 

 from before backwards, and pass through all their stages of growth 

 in that order of position, the anterior ones having their fangs and 

 alveoU completed, whilst the posterior teeth are lodged in a common 

 groove, or may be supported at the back part of the series by the 

 gum only. When the formation of the entire series of teeth 

 approaches its completion, the Dolphin resembles the Alligator in 

 having the anterior teeth lodged in sockets and the posterior teeth 

 in an alveolar groove. In the Cachalot the large middle teeth of 

 the series are the last to have the fang solidified. 



The calcification of the dentinal pulp in the Dolphin proceeds 

 so that the pulp-cavity extends to near the apex of the tooth for 

 a longer period than in the simple teeth of quadrupeds ; for a 

 certain time the teeth continue widely open at their base, and of 

 a simple conical form, as in the Crocodiles ; but, not being subject 

 to displacement by successors pushing them vertically from their 

 sockets, the calcification of the remaining pulp proceeds to close 

 the basal opening, and to form a fang which, with the exception 



(1) On WTiales, Philos. Trans. 1787, p. 398.— The abortive tusks of the Walrus are the 

 sole exceptions to this rule in the true Cetacea. 



