POLYGYRELLA-POLYGYRATIA. 81 



Aperture narrowly crescentic ; lip blunt, thickened within except 

 toward the upper termination, not in the least expanded, toothless; 

 parietal wall toothless. Type P. yatesi Coop., pi. 20, figs. 32, 33, 

 34. 



External characters and genitalia of animal unknown. 



Jaw low, wide, slightly arcuate, without median projection below ; 

 surface with a strong transverse line of reinforcement, and about 12 

 wide crowded ribs, denticulating either margin (pi. 36, fig. 12, P. 

 yatesi). 



Radula long and narrow; teeth after the formula 18.6.1.6.18. 

 Teeth like those of Polygyrella, but ectocone of marginals simple 

 (pi. 36, fig. 11, P. yatesi}. 



This group has been united with the European genus Gonostoma, 

 but erroneously. It is readily distinguished from that type by the 

 non-expanded lip and glassy texture of the shell. The dentition also 

 differs widely. The genital organs of Gonostoma present character- 

 istic features, but as the soft anatomy of Ammonitella is unknown, 

 no comparison can now be made. The American species will be 

 found to have the genitalia simple, as in Polygyrella, if my estimate 

 of its affinities proves to be correct. 

 P. yatesi Coop., iii, 115. Calaveras Co., California. 



yatesiana W. G. B., olim. 



Genus POLYGYRATIA Gray, 1847. 



Polygyratia GRAY, Proc. Zool. Soc. Lond., 1847, p. 173, type H. 



polygyrata. Ophiogyra ALBERS, Die Hel. 1850, p. 91, type H. 



polygyrata Born. Systrophia PFR., Malak. Blatter, ii, 1855, p. 136, 

 for H. helicycloides,systrophia, heligmoidea. Entodina ANCEY, Con- 

 chologists' Exch., i, p. 64, May, 1887, type H. reyrei. 



Shell disk-shaped, flat or nearly so above, concave beneath, com- 

 posed of 5-10 narroiv, closely coiled whorls, equally visible above and 

 beloiv, the last descending in front. Aperture oblique, rounded or 

 subtriangular, the lip generally narrowly expanded, sometimes 

 toothed ; parietal callus inconspicuous or raised into a tooth-like pro- 

 cess. Type H. polygyrata Born, pi. 20, figs. 37, 38. 



Animal unknown. The species are said to live in forests. The 

 typical subgenus is confined to the central portions of South Amer- 

 ica. The affinities of the genus are problematical. It may per- 

 laps prove to be allied to Polygyrella. 

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