PILSBRY: NON-MARINE MOLLUSCA OF PATAGONIA. 569 



embraces the whole La Plata drainage, together with some rivers flowing 

 directly into the Atlantic, but having their rise adjacent to or interposed 

 between the head streams of the Uruguay and Parana Rivers. Many new 

 forms doubtless remain to be found, since only an inconsiderable part of 

 the waters of the Plata system have been explored for mollusks. 



To what extent the specific characters of the forms vary from place to 

 place, we cannot say, since most of them are known from a single locality. 

 P. lapidum, which has been assigned a wide range, seems to vary with 

 locality, and probably several species will eventually be recognized in 

 what is now considered lapiditm. Of most of the forms, many specimens 

 have been studied, some of them by hundreds, and I have been astonished 

 at the absence of intermediate or ambiguous individuals, such as one finds 

 in the fresh-water Pleuroceratidce or Melaniidce. It is however well known 

 to those who have studied large quantities of fresh-water snails, that the 

 Amnicolidce are generally conservative ; the specific features are crystal- 

 lized, while in the Melanians they are fluid. 



SIGNIFICANCE OF THE CHARACTERS OF POTAMOLITHUS. 



In the Amnicolidce, as in the Viviparidce, the prevalent genera almost 

 everywhere are smooth-shelled forms with rounded or convex whorls. 

 Such forms have prevailed since the first appearance of these families. In 

 those Amnicolidce which are sculptured in the adult stage, the early 

 (embryonic and early neanic) stages are smooth or nearly so. These facts 

 apparently point to the conclusion that smooth, rounded shells are prim- 

 itive and sculptured shells derivative in these families. 



Throughout Neocene time, carinate, varicose or otherwise strongly 

 sculptured species or genera of these families have frequently appeared, 

 but their distribution has been local and their duration brief. In some 

 cases the genesis of these sculptured or distorted forms from smooth and 

 normal types has been traced, as in the case of Viviparns hoerncsi of the 

 Pliocene of southeastern Europe, and Viviparus altior and limnotlianma of 

 the Floridian Pleistocene. At the present time, sculptured Viviparidce 

 and Amnicolidce are comparatively rare and confined to small areas. 

 Margarya in Lake Tali, Tulotoma in the Coosa River, Pyrgitlopsis in 

 Pyramid Lake, Nevada, and Tryonia in a very restricted area in the 

 Southwest, are familiar examples. To these may be added the group of 

 carinate species of Potamolithus in the Uruguay River. In all of these 



