CETACEA. 333 



extreme case, the number of primary pieces is constantly 

 seven.* 



§ 2. Cetacea. 



Remarkable exemplifications of the law of uniformity 

 of organic structure are furnished by the family of the Ce- 

 tacea, which includes the whale, the cachalot, the dolphin, 

 and the porpus, and exhibits the most elementary forms of 

 the type of the mammalia, of which they represent the earl)^, 

 or rudimental stage of development. Here, as before, we 

 have to seek these first elements among the inhabitants of 

 the vvater: for whenever, in our progress through the ani- 

 mal kingdom, we enter upon a new division, aquatic tribes 

 are always found to compose the lowest links of the ascend- 

 ing chain. Here, also, we observe organic development 

 proceeding with more rapidity, and raising structures of 

 greater dimensions in aquatic than in terrestrial animals. 

 The order Cetacea comprises by far the largest animals 

 which inhabit the globe. Whatever may have been the 

 magnitude of those huge monsters which once moved in 



* The Bradypus tridadylus, or three-toed sloth, was, till very lately, 

 thoug-ht to constitute a notable exception to this law, being- described as 

 having- nine, instead of seven, cervical vertebrae. It is now found, however, 

 that the two last of these vertebrae, which appeared to be supernumerarv, 

 ought properly to be classed among- the dorsal vertebrae, of wiiich they possess 

 the distinctive characters, not only from the form and size of tiieir transverse 

 processes, but also from their having- small bony appendices, articulated with 

 them by a reg-ular joint at their extremities, and corresponding exactly, both 

 in shape and situation, to the ribs, of which they may, in fact, be considered 

 as rudiments. These small bones have been observed, both by Meckel and 

 by Cuvier, attached to the ninth vertebra: and Mr. T. Bell has recently not 

 only confirmed the observations of these anatomists, but has farther discovered, 

 that similar rudimental ribs are attached also to the eighth vertebra. (See 

 Philosophical Magazine, third series, iii. 376.) The Brudypiis forquaius, 

 which has been said to possess eight cervical vertebrae, will, perhaps, on closer 

 examination, be hereafter found not to deviate, any more than the three-toed 

 sloth, from the normal type, as reg-ards the number of these vertebrx. In- 

 stances have occiu'red of supernumerary cervical processes, or ribs in the hu- 

 man skeleton. (See Edinburg-li Medical and Surg-icid Journal, xl. 304.) 



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