76 STUDIES IN GEOLOGY, No. 3 



are sometimes noticeable. In most specimens growth-lines 

 are evident on all the whorls ; in the lower whorls they are 

 imbricate, the growth-periods being set off. Base rounded, 

 the carina becoming obsolete near the peristome. Dimen- 

 sions of fragment of largest specimen: length of 3 whorls, 

 57 mm.; maximum diameter, 26.5 mm. A fragment bear- 

 ing 5^2 whorls is 78 mm. long, with a maximum diameter 

 of 25 mm. 



The generally central position of this species in the group 

 of related forms has suggested giving its name to the group. 

 The comparatively large amount of material in the Nelson 

 and Hopkins collections shows somewhat diverse varia- 

 tion, particularly in sculpture, and it has been difficult to 

 discern those characters which might rationally be held to 

 carry specific weight ; however, the salient features men- 

 tioned in the discussion of the group seem to express specific 

 relationship, and at all events are certainly constant and 

 recognizable in the suite of specimens studied. The number 

 and position of the subsidiary threads is by no means con- 

 stant for the species as here delimited; the outline of the 

 whorls and the primary sculpture seem to offer the best 

 criteria for recognition. 



T. nelsoni was described by Nelson (loc. cit.) as T. sutu- 

 ralis, but, that name being preoccupied, it is necessary to 

 propose a new one. In the group of specimens labelled 

 suturalis by Nelson were a half-dozen or more of rotundata 

 Grzybowski. Nelson's description, though too brief and in- 

 exact, unaccompanied by a figure, shows that he drew no 

 distinction between nelsoni and rotundata. Grzybowski 

 apparently had no true nelsoni in his collection. 



The young forms are described separately from the adults 

 on account of the possibility that they may be of different 

 species. The writer hesitated some time before deciding that 

 they should be united, and although all evidence now avail- 

 able seems to require such procedure, there are important 

 arguments for their separation. The smaller forms have the 



