80 POLYGYRELLA. 



Acad. Nat. Sci. Phila. 1890, p. BQO.Adelodonta ANCEY, Le 

 Naturaliste, Dec., 1880, p. 334, type H. polygyrella. 



Shell disk-shaped, the spire flat or nearly so, periphery rounded, 

 even in the young ; umbilicus wide within, showing all the whorls ; 

 texture somewhat glassy and subtranslucent, polished beneath ; color 

 yellow, greenish or light brown ; whorls 6-8, narrow, slowly widen- 

 ing, the last slightly descending in front. Aperture subtnangular, 

 oblique ; peristome blunt, not expanded, thickened within, with or 

 without lip-teeth or internal teeth ; parietal wall bearing an erect 

 triangular tooth. Type P. polygyrella, pi. 31, figs. 28, 29, 30, (see 

 also pi. 31, figs. 33, 34, 35, P. harfordiana, enlarged). 



Jaw very wide, arcuate, without median projection below ; surface 

 with numerous (17-36) broad, slightly separated ribs, denticulating 

 either margin (pi. 31, fig. 32, P. polygyrella). 



Radula long and narrow, with teeth according to the formula 22. 

 5.1.5.22. Teeth as in Polygyra, the centrals tricuspid the mesocone 

 longer than the basal-plate ; laterals bicuspid, marginals bicuspid, 

 the ectocone bifid on the outer teeth (pi. 31, fig. 41, P. polygyrella*). 



Genital system like that of Polygyra, but having the duct of the 

 spermatheca rather longer (pi. 31, fig. 31, P. polygyrella). 



This group agrees with Polygyra in essential features of dentition, 

 jaw and genitalia; it differs from that group in the glassy texture 

 of the shell and its totally unreflexed lip. The texture of the shell 

 is like Systrophia, but that South American type has the lip-edge 

 slightly expanded. 



P. polygyrella Bid. & Coop., iii, 129. Coeur d'Alene Mts., Idaho. 

 P. harfordiana Coop, iii, 130. Fresno Co., California. 



Subgenus AMMONITELLA Cooper, 1869. 



Ammonitella J. G. COOP., Amer. Journ. of Conch, iv, p. 209. 

 (Issued February 4, 1869). Gonostoma W. G. BINNEY, Terr. Moll, 

 v, p. 261. 



Shell Ammonite shaped, with sunken, concave spire, and open 

 umbilicus showing all the whorls; periphery broadly rounded; 

 texture glassy, subtranslucent ; whorls about 6, very narrow and very 

 closely revolving, the last whorl embracing the greater part of the pre- 

 ceding, deflexed in front, its suture somewhat tangentially produced. 



