Paleontology Zorritos Formation 67 



T. aiicincta in that respect. Its general relationships are 

 similar to those of filicituta. 



Upper Zorritos. Mouth of Ouebrada de las Alturas. 



Group of TURRITELLA XELSONI 



Twelve of the species and varieties of Turrit ella in the 

 Zorritos fauna form a natural group whose internal rela- 

 tionships are sufficiently interesting morphologically and 

 probably genetically to make it better worth while to con- 

 sider them together than to leave the discussion of their 

 differential characteristics to the isolated remarks normally 

 accompanying each specific description. The group is desig- 

 nated here the "group of Turritella nelsoni' as much because 

 of the apparent abundance of that form in the fauna as 

 because of its central or typical position in the group as 

 here presented. The species and varieties which constitute 

 the group are : 



Turritella inca Grz. 



•' inca var. trita n. var. 



" nelsoni n. nom. 



" nelsoni var. rotundata (Grz.) 



" nelsani var. trullissatia n. var. 



" infracarinata Grz. 



" infracarinata var. zorritensis n. var. 



" prenuncia n. sp. 



" prenuncia var. inconspicua (Grz.) 



" robitsta Grz. 



" rohusta var. abrupta n. var. 



" charana n. sp. 



Progressive development in the group is characterized by 

 changes along two different lines, one from bicarinate con- 

 vex forms (inca, trita) to rounded convex forms (rotundata, 

 inconspicua, zorritensis) and the other from the bicarinate 

 convex forms (inca) through monocarinate forms with the 

 supracarinal surface flat, conical (trullissatia, infracarinata, 



