372 A MANUAL OF AMERICAN LAND SHELLS. 



gans, Say. It is readily distiuguislied from that and all other described 

 species by the umbilicus, broad at the commencement and rapidly 

 narrowing beyond the second whorl, with the peculiar groove visible in 

 all the whorls of the umbilicus, of the same character as that noticed 

 by Say in auriculata, though deeper. 



The name tholus is derived from the resemblance of the slightly 

 raised, rounded spire to a low dome. 



Jaw with about 15 adjoining, broad ribs, denticulating either mar- 

 gin. 



The lingual membrane of Mooreana (Terr. Moll., V, Plate VI, Fig. Q) 

 has 20-1-20 teetli, with 8 laterals. There are two transition teeth, with 

 simple inner cutting points. 



Genitalia not examined. 



Polygyra hippocrepis, Pfeiffer. 



Shell rimately perforated, depressed, rather heavy, closely striated, 

 Fig. 407. opaque, smoky ; spire flattened ; suture impressed ; whorls 

 '\^h 5 J, narrow, scarcely convex, the last subcarinated above, 

 more convex below, falling abruptly at the aperture, and 

 behind it very much contracted and with a prominent iso- 

 lated bulge ; umbilicus at first expanded and grooved, but 

 rapidly terminating in a minute perforation ; aperture 

 almost horizontal, ear-shaped, riugent, complicated with 

 p. hippocrepis. tccth ; peristome white, thickened, its extremities joined 

 by an elevated, sharp, angular ridge, from which protrude far within 

 the aperture two laminre (the ui^per one sharper and more prominent) 

 the connecting terminations of which within the shell resemble a horse- 

 shoo ; the up])er portion of the peristome is slightly reflected and fur- 

 nished with an oblique entering angle, and the basal portion is callous 

 aud reflected ; an internal transverse tubercle on the base of the shell. 

 Greater diameter 12, lesser 10""" ; height, 5""". 



Helix Ixippocnpis, Pfeiffer iu Eoemer's Texas, 455 (1849); inZeitsch. fiir Mai., 1848, 

 119 ; Moil. Hel. Yiv., iii, 267 ; in Chemnitz., ed. 2, ii, 333, pi. cxxxi, figs. 4~ 

 6.— Reeve, Con. Icon., No. 1238 (1854).— W. G. Binney, Terr. Moll., iv, 77, 

 pi. Ixxviii, fig. 19 ; L. &. Fr.-\V. Sh., i, 96, fig. 172 (1869). 



DwdaJochila (?) hippocrepis, Tryon, Am. Joiirn. Conch., iii, 68 (1867). 



Pohj<j\jra hipjiocrcpin, \V. G. Binney, Terr. Moll., v, 273. 



Texan Subregion, at New Braunfels, Tex. 

 Animal not observed. 



