332 NOTES ON PROSOBRANCHIATA, IV., 



to be somewhat futile. Granted that the torteconch evolved 

 from a planoconch (such as Patella) and that we had the complete 

 series to study, then would we be unable to draw the line 

 between planoconch and torteconch; or granted that the torsion 

 was inherited directly from the molluscan ancestor and that we 

 had the complete genealogical tree, then would we debate as to 

 which was mollusc and which ancestor. 



iv. — The Systematic Value of the Protoconch. 



This is a question that is coming into greater and greater pro- 

 minence, and one worthy of our closest attention; kept within 

 its proper limits, it bids fair to be a most valuable character 

 in arriving at a natural classification of the minor groups of the 

 Gastropoda. There seems, however, to be a growing inclination 

 to overestimate its value. 



If the protoconch is a veloconch, then must we not lose sight 

 of the fact that it was formed by the mollusc at the age when it 

 had only just assumed a definite Gastropod character, that is the 

 mollusc was so young that it had not as yet assumed generic 

 characters, therefore the protoconch can hardly be reasonably 

 used as a generic character. The same applies to an ananeano- 

 conch moulded inside a veloconch. An ananeanoconch such as 

 that of Megalatractus, which was formed independently of the 

 veloconch, will, however, betray generic characters, as would the 

 mollusc itself in the same (ananeanic) stage. 



I concur with Grabau's contention(l) "that the protoconchs of 

 all species within a given genus should agree as to their essential 

 characters, and that no species can be congeneric in which the 

 protoconchs show a radical difference," (the italics are nnne) but 

 I maintain that it has never, and believe that it never will be 

 shown that there is a really essential deep-seated difference 

 between the protoconchs of moUusca generically allied as to their 

 shells and anatomy, unless the said protoconchs be ananeano- 

 conchs. 



The question hinges on what is to be considered an essential 

 or radical difference. Let us review one or two instances. Sup- 



