POLYGYRA. 



275 



same purpose in the economy of the animal, as the " fulcrum "originally noticed 

 by Mr. Li-a (Observations, Vol. V. p. 80) in Slrenotrema spinosutriy though of a 

 difl'oivnt construction. 



Jaw slightly arcuate, long, low, with about 20 ribs on the anterior surface, 

 crcnulatiug either margin. 



P. /asti</ati.s (IM. VI. Fig. II) has 21—1—21 teeth, with 8 laterals on the 

 linorual membrane. 



Polygyra Jacksoni, Bland. 



Shell narrowly umbilicate, depressed, shining, dark or pale horn-colored, 

 little elevated above, striated, convex beneath, with finer almost obsolete striae ; 

 whorls 6, slightly convex, gradually increasing, the last sud- 

 denly deflected, contracted and above gibbously inflated behind 

 the aperture ; suture impressed ; aperture oblique, lunate- 

 circular, with 3 teeth ; peristome thickened, brownish-red, 

 shortly reflected, with the scarcely approaching margins joined 

 by a white, linguiform, bicrural, deeply entering tooth, the 

 basal margin with a strong, oblique, sinuous fold, the right 



with a deeply seated 

 mill. : heiuht, 4 mill. 



tooth. Greater diameter 7, lesser 6 



p. Jacksoni. 



Helix Jacksoni, Bland, Am. Journ. Conch., II. 371, PI. XXI. Fig. 8 (1866). — 



W. G. BiNNEY, L. & Fr.-W. Sh., I. 98, Fig. 174 (1869). 

 Dcedalocliila Jacksoni, Tryon, Am. Journ. Conch., III. 67 (1867). 



Fort Gibson, Indian (Cherokee) Territory ; Springfield, Mo. I am inclined 

 to rank it among the species of the Texan Subregion. 



This species belongs to the same group as, and is most nearly allied to, P. 

 Hazardi, Bland (P. plicata, Say), from which, however, it may be readily dis- 

 tinguished by the very different character of the parietal and basal teeth. 

 Tliis species has no internal tubercle. 



Animal not observed. 



Fig. 175. 



Polygyra Troostiana, Lea. 



Shell riraately umbilicated, discoidal, slightly convex above, flat- 

 tened below, obtusely carinated, with separated strong- rib-like 

 striaj throughout,^ hirsute, russet horn-color; spire not much ele- 

 vated; whorls 5^, flattened, the last more convex, descending at 

 the aperture, grooved behind the peristome, with a smoother 

 bulge, below plane, widely rimated, and ending in a small umbili- 

 cus ; aperture oblique, subrcniform, very much contracted, far 

 Q___^^ within on the base of the outer whorl wirh a small, detached, 

 P. Troostiana, erect, roundcd tubcrcle ; peristome white, thickened, continuous, 

 ends approached, joined by an excavated, emarginate, somewhat 



enlarged. 



1 Some of the strife extend over the carina on to the base of the shell without beiug car 

 ried into the umbilicus. 



