404 HISTORY OF GEOLOGY AND PALEONTOLOGY. 



1875 attempted to sub-divide Ammonites into a number of 

 families and genera, but his attempt only served to show how 

 extremely difficult it was to give a precise definition and limit 

 to the individual groups of forms. All groups seemed con- 

 nected with one another by intermediate types. 



Neumayr therefore fell back on the genealogical principle 

 as the guiding feature in his classification, and combined into 

 narrower or wider groups all those forms which in his opinion 

 were either nearly related or directly connected in the line of 

 descent. 



Previously to Neumayr, Waagen (1869) had traced the 

 genealogical tree of the species Ammoniles subradiatus through 

 several stratigraphical horizons, and had proposed the term 

 " mutation " to signify the insignificant variations or modifica- 

 tions apparent in the members more remote from one another 

 in time. The stronger emphasis placed on the phylogenetic 

 relationships introduced a more speculative and subjective 

 character into the study of Ammonites, but it also gave an 

 incentive to a more detailed investigation of the shell develop- 

 ment and to a comparison of the ontogeny and phylogeny of 

 these organisms. 



Hyatt had endeavoured in the year 1872 to find out the 

 approximate "embryology" of the Ammonites by an ex- 

 amination of the primary chambers and the innermost coils 

 of the shells, and had by this means been able to verify the 

 essential difference between the embryonic development of 

 Nautilidae and Ammonitidse which had been stated by Bar- 

 rande and Saemann. In 1880, Wiirtenberger emphasised 

 the agreement in the evidences of ontogeny and phylogeny 

 regarding the shell development in the group of Ammonites. 

 Meunier-Chalmas observed (1873) a striking resemblance of 

 the embryonic chambers of certain Ammonites with Spirilla, 

 and argued that a near relationship existed between the Am- 

 monites and the Dibranchs. Upon other grounds, Gray, 

 Suess, and to a certain degree also Quenstedt, formed a similar 

 inference; and Steinmann in 1890 expressed his opinion 

 that the genus Argonauta was a lineal descendant of the 

 Ammonites. The development of the chambered shells of the 

 Cephalopods was made the subject of a masterly and compre- 

 hensive series of researches by Branco (1881), and led this 

 observer to apply the character of the embryonic chambers 

 as a basis for the chief sub-divisions of the Ammonitidce. 



