[ 108 ] 



POLYGYRA, Say. 



This is a tropical genus, containing many flat, wheel-shaped 

 species Avhich inhiibit the shores of the Gulf of Mexico, the West 

 Indies and Mexico. The southern limit of the group extends to 

 the vast regions of the empire of Brazil, whence come the curious 

 P. heligmoidea and P. helicycJoidea. Unlike Pcedalochila, the 

 aperture is simple and small, the lip reflected but not expanded, 

 and not dentate, while the parietal tooth is small. The aspect 

 of the shell is singularly like that of Pla7iorbis, and reminds one 

 strongly of several of the species of that fresh water genus in- 

 habiting Europe. 'A thin, thread-like lamina occasionally re- 

 volves upon the inner wall of the aperture, and is visible through 

 the whorl ; it has been detected in most of the species, and 

 probably is at times developed in all of them, though many spe- 

 cimens are without it. 



1. Polygyra anilis, Gabb. 



Plate 11, figures 17, 18. 



Spire nearly flat, whorls 4^^ the last descending to the aper- 

 ture, and a little constricted, suture Avell impressed, surface 

 microscopically striate above and below ; base showing about IJ 

 whorls, with a minute central perforation. White. 



Diam. 13, height 6 mill. 



Guaymas, Mexico. 



This is scarcely a typical Polygjjra. as 'it does not exhibit so 

 many whorls on the base as the other species. The aperture, 

 however, is that of a Polygyra. 



2. Polygyra cereolus, MUhlfeit. 



Plate 11, figures 19, 20, 21. 



Shell lenticular, the spire very nearly flat, under surface flat 

 or slightly concave, periphery subangulate; whorls 7, flattened 

 and closely ribbed above ; base smoother, showing 5 whorls, with 

 a narrow umbilicus ; aperture small, subtriangular, the margins 

 connected by a slight callus, developing in the middle into a 

 small triangular tooth. Light horn color. 

 Diam. 14, alt. 3'5 mill., \ ay. major. 

 " 9, " 2-5 "■ " minor. 

 '/ " 3 East Florida. 



Differs from P. septemvolva principally by its umbilicus being 

 much narrower. (Compare fig. 19 with fig. 22.) 



