62 THE CONSTITUTION OF THE GASTROPOD PROTOCONCH, 



organised mollusc, so that, if represented without modifica- 

 tion in the recent fauna, its representatives would probably 

 form a group of similar value to the Thecosomatous Pterc- 

 poda.) 



Notwithstanding their phylogenetic immutability, the 

 Veliger and indeed also the Trovhosphere larvae present quite 

 obvious differences among themselves, as also do their X ecto- 

 conchs ; and the questions naturally arise: To what are these 

 differences due ? And are they not taxonomically significant ? 



The second question must be answered in the negative, cer- 

 tainly as concerns the N ectoconch , and probably also the larva 

 within it. Since tachygenesis impressed the Yel'ujer upon 

 the ontogenesis, its waves have broken against a rock of 

 resistance to variation so strong at this stage that fundamen- 

 tally all the larvae are alike. Phylogenetic factors have not 

 acted upon the stage, for we do not find several types corres- 

 ponding to the several groups of the Gastropoda. The cause 

 of the variations of the larvae must then be sought for among 

 other factors than phylogenetic, and, if this be so, the varia- 

 tions are not of taxonomic significance. 



One of my critics has said (7): "No recent Triton has 

 a protoconch*^, such as is figured for this species {T. radialis 

 Tate), by Mr. Kesteven (28), and I consider that it should be 

 definitely separated from the species he has associated it with 

 on this account alone, and most of the Australian Tertiary 

 species will naturally fall into the same group. ' ' This dictum 

 apparently affords a very striking illustration of the errors 

 that may arise out of giving undue taxonomic value to the 

 Protoconch. The writer in question was apparently under 

 the impression that I had classed Triton radialis with recent 

 species. Cossmann (6), one year later than myself, had 

 proposed a new genus, .1 vstrnfrifon, for the Tertiary forms 

 referred to, and made T. radialis the type of the 



• Pseudo-iiectoconch— a oast of the Nectoconch, and formed during later 

 stages than Phylembryonic. 



