INTRODUCTION. 



■whether any particular phenomenon observed is of value to the 

 systematist, or not. A phenomenon may be of the highest 

 importance in the classification of certain genera, and be of little 

 value from that point of view in reference to others. The study 

 of the development of the Gasteropoda would show better than 

 anything else what value is to be ascribed to any particular 

 structural feature in the shell, in that sense ; but little has hitherto 

 been attempted in that direction. 



An exhaustive study of auxology cannot, obviously, be 

 chronicled in a Catalogue like the present; but I have seized 

 this opportunity of selecting the most promising species and 

 genera of Australasian Tertiary Gasteropoda from' the material 

 in the Museum, and, as far as is consistent with the plan of the 

 book, have detailed certain structures of general interest which 

 may hereafter prove of service to workers in elaborating the genesis 

 of the modern Gasteropod fauna of the antipodes. 



To the geologist, also, particulars concerning the growth of these 

 Tertiary shells are not without their significance. It is very 

 difficult for paleontologists, by the methods now commonly adopted 

 in classifying the Mollusca, to say how much of a variation from 

 a normal type shall constitute a species : the personal equation 

 is always with us. And seeing that Tertiary beds are commonly 

 classified according to the relative percentage of extinct species 

 of mollusca found in them, and that for the reason just given 

 it is almost impossible to arrive satisfactorily at such percentages, 

 it seems to me that, rightly understood, the phylogeny of that 

 division of the animal kingdom is a broader and surer basis for 

 classification of the various horizons, and might be made to run 

 pari passu with the Lyellian method. Its most useful application, 

 no doubt, would be in reference to homotaxis. This suggestion is 

 to a large extent prompted by the great difficulty which geologists 

 experience in working out the synchrony of the Tertiary beds 

 of Australia, to this day a source of much controvei'sy between 

 them. 



As instances of the application of phylogeny in matters of this 

 kind let me refer to pp. 17, 18, where in reference to the so-called 

 Pteropoda it will be noted that the assemblage of genera found 

 in certain rocks classified by competent authorities as Eocene is 

 not such as might have been expected in rocks of that age, when 

 the known history and origin of the Pteropoda are taken into 



