656 STEARNS 



sute periostracum. Whorls six to six and a half, rounded and grad- 

 ually increasing. Suture quite distinct. Aperture rounded lunate. 



Of the three specimens submitted for examination the description 

 is made from the most perfect. A measurement of these gives the 

 following figures : 



Altitude 0.63, 0.60, 0.52 inch; max. diameter 0.85, 0.85, 0.79 

 inch; apical angle about 95°. After a careful comparison with the 

 living Helices of the West Coast, I find that while the above are larger 

 than any examples of P. cohunbiana that I have seen, and differ 

 from that form in the umbilical feature, and in having a less reflected 

 lip, in other respects they exhibit many of the characters of the latter, 

 as in the following particulars: General facies (size and umbilicus 

 excepted) ; apical angle, when compared with elevated individuals of 

 P. Columbiana ; number of whorls and convexity of the same ; su- 

 tural definition ; ratio of increase in size of whorls ; basal convex- 

 ity ; and finally in shape of aperture. 



P. dalli suggests an ancestral form, from which have proceeded 

 such living representatives as are known by the specific titles coliiyn- 

 biana^ devia^ etc., and relationship with P. townseiidiana^ its so- 

 called variety P. ptychophora^^ P. thyroides^ an eastern form whose 

 western range as reported to this date is Minnesota, and with P. pty- 

 chophora which has crossed the Cascade Mountains, and extends 

 easterly to the Deer Lodge region of Montana. 



All of the species named are rather of an eastern than a West Coast 

 type. One aspect of columbiaua^ labiosa^ has also been found in 

 Deer Lodge Valley. 



The wide gap between the western range of the eastern form, and 

 the eastern distribution of the western species, may some time yield 

 to the investigator connecting links, in the specific or varietal chain, 

 indicating the lines of migration as well as descent. The Museum 

 number of P. dalli \% 13401. 



Genus Polygyrella Binney 1863. 

 Subgenus Ammonitella Cooper 1869. 



AMMONITELLA YATESI PRECURSOR Stearns. 



(Plate XXXV, figs. 8, 9, 10, 11, 12.) 



Gonostoma Kz/^5/ Cooper (Stearns) Bull. 18, U. S. Geol. Survey, p. 16, pi. 

 Ill, figs. 8, 9, 10, II, 12, 1885. 



The Cope-Condon collections contain four specimens (Mus. no. 



1 The dental formula in these two species is nearly the same according to 

 Binney ; this character is, however, not a conclusive one. 



