Paleontology Zorritos Formation 69 



prenuncia)^ to forms with one prominent anterior carina and 

 the main surface of the whorls concave (robusta, charana 

 abrupta). The morphologic relationships of the members 

 of the group to one another may be pointed out most 

 clearly by the use of the accompanying diagram. The 

 arrangement of the species in the diagram has not 

 been made with the idea of expressing consistent 

 genetic relationships, and as a whole it could hardly be 

 regarded as a phylogenetic table; but it is not impossible 

 that it may indicate something of the general ancestr\' of the 

 different species, and if we disregard the direction in which 

 evolution may have proceeded, portions of the arrangement 

 seem to express most likely genetic relationships. 



The variety trita is distinguishable from T. inca by the 

 obsolescence of both the posterior carina and the subsidian.- 

 threads above the main carina. In trita the space corres- 

 ponding to the band in inca bears no prominent threads, and 

 the band is purely vestigial, the upper carina being faintly 

 distinguishable. In nelsoni the upper carina is lost, and there 

 is a varied development of the interstitial threads, with a 

 tendency in some forms toward rounding of the anterior 

 carina. Such partially rounded forms verge on Jhe variety 

 rotundata, and although they represent a partially expressed 

 transition, a large number of specimens shows that the two 

 are distinct, that the carina is constantly present in nelsoni, 

 even though it is less sharp in some forms than in others. 

 The differential characters of T. nelsoni are the single carina 

 at the anterior fourth of the whorl-length, the slightly con- 

 vex supracarinal surface with four primary and varying 

 subsidiary threads, and the sutural indentation formed by a 

 short, sharp bevelling of the posterior part of the whorls and 

 the longer pre-carinal surface of the succeeding whorls. 



Grzybowski's figure of T. inca depicts the earlier whorls 

 as more rounded than the later ones, which bear the charac- 

 teristic band. A curious reversal of this characteristic is 



