70 STUDIES IN GEOLOGY, No. 3 



found in nelsoni, the earlier whorls of which are distinctly 

 banded similarly to inca, with the later whorls rounded. 



The neanic whorls of inconspicua are monocarinate in- 

 stead of bicarinate, as in the young of nelsoni, and the sculp- 

 tural threads are more widely spaced. The adult whorls of 

 both are rounded, but the difference in sculpture of incon- 

 spicua is easily recognizable as constant. The difference be- 

 tween T. prenuncia and its variety inconspicua parallels that 

 between T. infracarinata and its variety zorritensis; pre- 

 nuncia is a carinate, slightly rounded form, with straight- 

 sided whorls, and inconspicua has the whorls well rounded. 

 The sculpture of the two is practically the same. The early 

 neanic whorls of inconspicua are not known, but those parts 

 of the spire available indicate that it is probably somewhat 

 like that of form (a) of prenuncia. (see systematic discus- 

 sion of prenuncia.) 



The forms which branch off diagrammatically from 

 nelsoni in the other direction, towards flat-angular and con- 

 cave-angular outlines, are of two series, the first of which 

 (trullissatia, infracarinata \ prenuncia) is closely related to 

 T. nelsoni, being different chiefly in having the supracarinal 

 surface flat, and the second of which (robusta, charana, 

 abrupta) does not show relationships as clear as the fore- 

 going, but which nevertheless seems to represent a continu- 

 ation of the general type, the common difference lying in the 

 concavity of the whorls above the suture. 



The variety trullissatia is distinct from T. nelsoni, on one 

 side, through its flat supracarinal surface, with a generally 

 sharper carina; on the other side it differs from infracarin- 

 ata in the sculpture, the interstitial threads being absent, and 

 five instead of four prominent threads occupying the supra- 

 carinal surface. The variety trullissatia is further distinct 

 from nelsoni in the absence of a sutural notch, tendency 

 towards greater diversification of sculpture, and greater 

 coarseness of the primary and secondary sculpture. T. in- 

 fracarinata apparently represents a development of trullis- 



