570 PATAGONIAN EXPEDITIONS : ZOOLOGY. 



cases, the distribution of the forms is very restricted, often to a single lake, 

 or a few springs; and in some cases, as Pyrgtdopsis and Tryonia, we 

 have evidence that the species had a wider range in the Pleistocene and 

 are now apparently approaching extinction. 



The facts indicate that in Amnicolidce and Viviparidce, shell-sculpture 

 is a phylogerontic character, showing the approach of senility of the race ; 

 strongly developed sculpture in a species signalizes its last incarnation. 



The considerations advanced above go to show that the present Pota- 

 molithus fauna consists in large part of species which in an evolutionary 

 sense are aged, are more or less distinctly gerontic or senile. Over 80 

 per cent, of the species have characteristics which indicate, as experience 

 has shown, that they represent side lines of evolution, impotent to continue 

 the phylum, or to give birth to new phyla. There remains also a small 

 group of unspecialized species represented by P. lapidwm of the Parana 

 and its allies in southern Brazil. 



I have been unable to find a shred of evidence to connect the develop- 

 ment of sculpture in these fresh-water snails with the concentration or 

 increased alkaline content of waters they inhabit, as some conchologists 

 have assumed. It is doubtful whether any such modified forms inhabit 

 alkaline or saline waters, while it is positively known that most of them 

 do not. Amnicolidce which live in brackish or sea water are not strongly 

 sculptured, but as smooth as their congeners in fresh water. Examples 

 of this are found in certain species of Paludestrina (P. minuta (Tott), P. 

 acuta (Drap.), P. salsa Pils.) and Littoridina (L. aitstmlis (d'Orb.), etc.). 

 It is extremely likely that these are forms of fresh-water origin, which have 

 become adapted secondarily to more or less saline waters. Part of them 

 live also in perfectly fresh water. 



INTERRELATIONS OF THE SPECIES OF POTAMOLITHUS. 



The species now known belong to four collateral phyla, each compris- 

 ing forms in several very diverse stages of specialization. The less dif- 

 ferentiated species in each phylum retain in adults a globular or Naticoid 

 shell without keels or angles, and in three of the groups have the lip 

 simple and unspecialized. This type of shell is common to other genera of 

 the subfamily. In all of the phyla some species have developed ortho- 

 genetically a varix or crest at the lip ; the shape of the shell is profoundly 

 altered by spiral keels in some species. These modifications are more or 



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