PA^ONTOLOGY ZORRITOS FORMATION 71 



satia with the strengthening of the chief tertiary thread 

 behind the carina to secondary rank and the obsolescence of 

 the subsidiary sculpture. It is like trullissatia in shape, and 

 different in sculpture. The relationship is sufficiently close 

 for the consideration of infracarinata as a variety of trullis- 

 satia, but since the latter is itself a variety of nelsoni, and 

 infracarinata is too far removed from nelsoni to be consid- 

 ered as a variety of it on a plane with trullissatia, the rela- 

 tionship can not be truly expressed through taxonomic 

 scheme, and it is necessary to retain infracarinata in the 

 status of a species. The variety trullissatia has the sculp- 

 ture of nelsoni, but differs in shape ; T. infracarinata has the 

 shape of trullissatia } but differs in sculpture; infracarinata 

 is thus different in both characters from nelsoni. The 

 ground plan of the sculpture is similar for all the forms of 

 the nelsoni group, and in those forms which do not bear 

 subsidiary threads the seven or eight primaries remain 

 rather constantly. 



The variety zorritensis of T. infracarinata differs from 

 the true species in its rounded whorl-outline. The sculpture 

 is closely similar. In form and ground-plan of sculpture 

 zorritensis is similar to nelsoni, and thus represents a con- 

 necting link between infracarinata and nelsoni; however, 

 it lacks the diversified sculpture characteristic of nelsoni 

 and its varieties, and in consistent alignment of affinities it 

 belongs in a place close to infracarinata, whose relationship 

 to the other forms has just been reviewed. 



The last three members of the group, robusta, var. 

 abrupla, and charana, differ from their nearest relatives in 

 contour of the whorls, prominence of the carina, and sculp- 

 ture; there is thus less in common between these species 

 and the rest of the group than between other smaller 

 divisions. 



T. robusta differs from its variety abrupta almost solely 

 in the character of the sculpture, which consists of widely 



