216 ANIMAL PEDIGREES 



may be recognised, differing from one another much 

 as existing Crustaceans do in the relative size and 

 perfection of the three regions, head, thorax, and 

 tail, into which the body is divided. 



Evidence of a very different kind, and often of 

 far greater value, is afforded by the study of 

 shells, whether of Mollusca or of Foraminifera. 

 Such shells, like those of Orbitolites already 

 noticed, have no power of interstitial growth, and 

 increase in size can only be effected by the addition 

 of new shelly matter to the part already in 

 existence. In most instances these additions take 

 place in such manner that the older parts of the 

 shell are retained unaltered in the adult ; and 

 examination of the adult or fully formed shell will 

 then reveal the several stages through which the 

 shell passed in its development. In such a shell 

 for instance as Nautilus or Ammonites, the 

 central chamber is the oldest or first formed one, 

 to which the remaining chambers are added in 

 succession. If therefore the development of 

 the shell is a repetition of ancestral history, the 

 central chamber should represent the palseonto- 

 logically oldest form, and the remaining chambers, 

 in succession, forms of more and more recent 

 origin. 



Ammonite shells present, more especially in 

 their sutures and in the markings and sculpturing 

 of their surface, characters that are easily recog- 

 nised. Upwards of four thousand species are 

 known to us, of many of which large numbers of 

 specimens can be obtained, in excellent preserva- 



