HELIX. 81 



Greater diameter 8*, lesser 7, height 3 millimetres. 



Geographical Distribution. Found in large numbers by 

 Dr. Francis Moore in Washington Co., Texas. 



Remarks. It is difficult to express correctly in words 

 the specific differences of the various Polygyrce. This 

 shell combines the characteristics of several American 

 species. It has the spire of H. monodon Rackett, and the 

 columellar fold of Dorfeuilliana Lea, as figured Tr. Am. 

 Phil. Soc. vi. pi. xxiv. f. 118. The teeth are placed on the 

 inner edge of the peristome, as in Texasiana Mor., and the 

 curious carination at the umbilical region resembles that 

 of pustula Fer., which has not been noticed in descrip- 

 tions. 







HELIX THOLUS. 



PLATE LXXVIII. FIGURE 21. 



Testa solidiuscula, albida, nitens, costis obliquis notata ; spira rotun- 

 data, elevatiuscula, obtusa, tholiformis ; sutura valde impressa, anfr. 7, 

 convexiusculi, superi magis planulati, ultimus ad aperturam descendens, 

 obtuse carinatus, carina peristoma non attingens, post perist. canalicula- 

 tus ; infra carinam costae minus distinctas; basis plana, umbilicus latus, 

 perspectivus, anfractus eanaliculatos monstrans, quorum 2^ perspicue, alii 

 obscure videntur ; apertura perobliqua, semicircularis, ab axe remota ; 

 perist. album, latum, incrassatum, margine basali reflexiusculo, dentibus 

 duobus sinu rotundato disjunctis armatum; plica parietalis acuta, in me- 

 dio aperture projecta, margines peristomatis vix connectens. 



SYNONYMS AND REFERENCES. 



Helix tholus W. G. BINNEY, Proc. A. N. S. Phila. 1857, p. 186; Notes, p. 6. 



f 



DESCRIPTION. 



Animal not observed. 



Shell rather solid, white, shining, ribbed above, smoother 

 below ; spire obtuse, little elevated, rounded ; whorls seven, 

 convex, the upper ones more flattened, the last bluntly 

 carinated; carina not reaching the peristome; base paral- 

 lel to the suture ; umbilicus broad, half the larger diame- 

 ter of the shell, showing two and a half deeply grooved 



11 



