THE RECAPITULATION THEORY 301 



parallelism between the embryological and palaeon- 

 tological records, not merely for Echini, but for 

 other groups of Echinodermata as well. 



The Tetrabranchiate Cephalopoda are an excel- 

 lent group in which to study the problem, for 

 though no opportunity has yet occurred for study- 

 ing the embryology of the only surviving member 

 of the group, the pearly nautilus, yet owing to the 

 fact that growth of the shell is effected by addition 

 of shelly matter to the part already present, and to 

 the additions being made in such manner that the 

 older part of the shell persists unaltered, it is 

 possible, from examination of a single shell and 

 in the case of fossils the shells are the only part 

 of which we have exact knowledge to determine 

 all the phases of its growth ; just as in the shell 

 of Orbitolites all the stages of development are 

 manifest on inspection of an adult specimen. In 

 such a shell 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 palasontologically 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 recognised, and readily 

 preserved in fossils ; and the group consequently 

 is a very suitable one for investigation from this 

 standpoint. Wtirtenberger's admirable and well- 



