BY H. L. KESTEVEN. 69 



posed by Buckman and Bather, is the correct term to use. In 

 the Gastropod mollusc this stage is characterised by the 

 abortion of the velum, but in the majority of cases, the 

 reduction and disappearance of this organ is very rapid, or is 

 unattended by other variants, so that no recognisable con- 

 chylaceous record of the stage is left in the protoconch ; 

 nevertheless it probably corresponds in point of time with 

 the abrupt change which so commonly marks the protoconch 

 off from the true conch. Finally, it is concluded that, in most 

 cases, the early portions of the true conch are formed during 

 early juvenescence, that, in fact, they are Ananeanic struc- 

 tures. 



The Value of the Protoconch as a Taxonomic Feature 

 AND the Significance of some of its Forms. 



The value of any feature for taxonomic purposes depends 

 on its immunity from variation under circumstances which 

 phylogenetically may be regarded as extraneous ; or, stated 

 in another way, only those characters which reflect phylo- 

 genetic relationships may be safely used for taxonomic pur- 

 poses, and it is almost axiomatic that those most liable to 

 variation are the most untrustworthy. 



If it be found that any given feature has been so little 

 affected by the phylogenetic history of the organisms possess- 

 ing it, that it fails to show modifications corresponding to 

 the major groups into which those organisms are divisible ; 

 and if, further, it be shown that this character is plastic to 

 its environment, that feature is surely hardly trustworthy as 

 a guide to the division of the major into minor groups. Such 

 a feature as this is the Gastropod Protoconch. 



Perrier and Gravier (42) have maintained that Tachy- 

 genesis is the constant mode of action of heredity, that, in 

 effect, all continuous evolution consists in the development 

 of the new characters in successively earlier and earlier stages 

 in the ontogeny of the successive generations. The same idea 

 is contained in the law of Serres, which has been more 



