On the Classification of Mammalia. 173 



normal number ; shewing that at the outset the number was 

 of a subordinate value, as well as the relative signification of 

 the different kind of teeth. Nevertheless it can be distinctly 

 shewn that the three orders following, Rodentia, Insectivora, 

 and Carnivora, are synthetically combined and foreshadowed 

 in the group of Marsupialia, which, when considered zoolo- 

 gically in itself, cannot but strike any one as an odd group 

 standing isolated in the actual creation. 



§ 3. The order of Cetacea, the lowest amongst the normal 

 groups, may be subdivided into three families. The first and 

 lowest, the family of Balcenidce, is characterised by the ab- 

 sence of teeth, or, if not entirely absent, they have no func- 

 tion. These are the toothless or edentated cetaceans, remind- 

 ing us of the order of Edentata proper, our second prophetic 

 type. The second family, that of Physeteridae, exhibits well- 

 developed teeth on the lower jaw, and rudimentary ones on 

 the upper : the subdentated cetaceans of the authors.* The 

 third family, that of Delphinidai, seems to complete the pro- 

 gressive series in the development of teeth ; for the latter 

 exist here on both jaws, whence the name of ambidentated 

 cetaceans. The fourth family, that of Heterodontidw, in- 

 cludes the narwhal or predentated cetaceans, and some 

 other types in which the dentition is losing both its shape 

 and its function. The Monodon (narwhal) is closely allied 

 to Phocaina (porpoise), whilst Hyperoodon comes nearer to 

 Delphinus. The other genera are deviations or reminiscences 

 of the other families. Heterodonts, then, must follow the 

 dolphins in a natural and serial classification. The order of 

 Cetacea begins with the whales, and closes with heterodonts ; 

 the real superior groups are those placed in the middle, the 

 Delphinidse, which represent the normal cetacean type. They 



* Physeteridae, or sperm-whales, are more nearly allied to dolphins than to 

 whales, if we take into consideration the structure of the whole skeleton. We 

 might even say that Physeteridae are gigantic dolphins in which the develop- 

 ment of teeth has stopped, and the hody increased beyond all proportion. 

 That colossal mass which sperm-whales partake with the whales proper, is of 

 an incontestable inferiority, as it is unfit for graceful movements ; but, on the 

 other hand, the material strength is developed, and the muscular power in- 

 creased to harmonize with the immensity of the element in which they live. 

 Balaenidae, the lowest of the order, are likewise amongst the largest. 



