432 Zoological Society. 



closed, as I have already shown in the seventh of other Mammalia, 

 by a little osseous stylet extending between the under side of the 

 transverse process and the body of the vertebra, imitating the neck 

 of a true rib ; and as this is coexisting with the rudiment discovered 

 by Professor Bell, but here anchylosed with the end of the transverse 

 process, it really presents the appearance of the upper portion of a 

 true rib, merely having the neck a little thinner than usual. This 

 circumstance may perhaps seem to weaken my position ; but when I 

 consider that this vertebra presents the same general characters as 

 the seventh cervical of most Mammalia, where, although the rib be 

 wanting, the foramen is generally wanting also ; and also the exist- 

 ence of the rib together with the absence of the foramen in the 

 Polecat, I think the balance of evidence will still be in my favour. 

 And Professor Owen has shown to me, in the College of Surgeons' 

 Museum, a preparation from the human subject, showing a pair of 

 ribs articulated to the seventh cervical vertebra by head and tubercle, 

 just as are those of the true dorsal series. 



But it yet remains for me to notice one point of resemblance 

 between the ninth vertebra of the Sloth and the seventh of other 

 Mammalia, which seems to have escaped the scrutiny of Professor 

 Bell : that is, that the body of the vertebra is not rounded beneath, 

 as are those of the true dorsal series, but flat and square ; this flatness 

 resulting from the presence of a longitudinal ridge along each side 

 of its under surface, and seeming to represent in a rudimental form 

 the anterior flattened processes of the preceding vertebrae of the series, 

 and whose absence, noticed by Professor Bell in the ninth vertebra of 

 the Sloth, is equally characteristic of the seventh throughout the rest 

 of the class. 



At all events I think I have adduced, from the consideration of 

 the mammalian class alone, proofs of that truth which other depart- 

 ments of Comparative Anatomy have before so well established, that 

 Nature does not rigorously confine herself to those precise rules 

 which we lay down to account for her phenomena ; and also, that if 

 we do find it necessary to subdivide the spine into distinct regions 

 for convenience of description, we cannot do so by simply defining 

 characters taken from the peculiarities of a single species, but must 

 compare the characters which the vertebrae present throughout the 

 scale of beings, to ascertain which of them are the most constant 

 and most truly essential in their nature. We may at the same time 

 perceive, that the same artificial subdivision of the spine which answers 

 our convenience so nicely in one class, may be only partially, or not 

 at all, applicable in another ; since in Birds there are no lumbar ver- 

 tebrae, and one vertebra partakes both of the dorsal and sacral cha- 

 racter, while in Fishes we find no cervicals, and as ribs are appended 

 to all those of the abdominal series, neither lumbar nor sacral ver- 

 tebrae can be said to exist. 



However, with regard to the distinction between cervical and dorsal 

 vertebrae, as we see them in the class Mammalia, it follows, from the 

 remarks which I have made, that we can define it neither by speci- 

 fying any particular number as constituting the cervical series, nor 



