HOLOSPIRA. 83 



sculpta. The aperture in some specimens projected more 

 than in others which seemed fully adult. The internal arma- 

 ture consists of a short very wide flange near the base on the 

 pillar and a low but strong basal ridge extending about one- 

 third of a gyration slightly nearer the inner than the outer 

 wall of the whorls (Doll.) 



The internal pillar is about 1 mm. in diameter, of about 

 equal calibre throughout except in the earliest and last 

 whorls, and as usual in Holospira, shows fine whitish lines 

 on a gray- white surface ( bilamellatus, having two plates) . 



Section Bostrichocentrum Strebel. 



STREBEL, Beitrag zur Kenntniss der Fauna Mexikanischer 

 Land- und Siisswasser-Conchylien, iv, p. 80, for B. tryoni 

 Pfr. 



This group is characterized by the spiral lamella situated 

 low upon the axial column within the penultimate whorl. 

 There is also a low ridge or inflation spirally ascending the 

 column, which is not enlarged above. No other lamella or 

 folds exist. (Bostrychos, curl, and kentron, center). 



Three species with this structure are known: H. tryoni, 

 with 11-15 narrow, closely coiled whorls, and measuring, 

 length, 13.5, diam. 4.5 mm., to length 9.7, diam. 4 mm.; H. 

 veracruziana, with 17 whorls, length 17.5, diam. 5 mm. ; H. 

 pilsbryi, with 12-14 whorls, length 10.25-13 mm.; the whorls 

 higher and the shell more slender than H. tryoni. 



9. II. TRYONI (Pfeiffer). PL 22, figs. 52-58. 



Shell perforate and rimate, cylindrical, conic above, thin 

 but moderately solid, opaque white, sometimes a little stained 

 with brownish flesh-color, the cone usually brownish. First 

 iy 2 whorls smooth; following whorls densely, sharply and 

 finely striate; this sculpture continuing on the cylindrical 

 portion, but on the lower half becoming coarser and less 

 sharp, or irregular and almost obsolete, the last several 

 whorls being nearly smooth. Whorls 11 to 15, somewhat 

 convex, very narrow and closely coiled, 6 to 8 in the cylin- 

 drical portion, those above forming a rather short cone, the 



