186 PALEONTOLOGY. 



GASTEROPODA. 



LIMNMIDM. 



Genus ANCYLUS, Geoffroy. 

 Ancylus undulatus, Meek. 



Plate 17, figs. 12 a and 12 b. 



Ancylus itndulatus, Meek (1870), Proceed. Acad. Nat. Sci. Pliilad., 57. 



Shell thin, attaining a very large size, elliptic-oval in outline, being 

 sometimes slightly widest a little in advance of the middle ; apex much ele- 

 vated, pointed, curved backward, and placed about half-way between the 

 middle and the posterior margin; posterior slope concave; lateral slopes 

 neai'ly straight; anterior slope distinctly convex. Surface marked with fine, 

 rather obscure lines of growth, and strong, comparatively large, concentric 

 undulations, most distinct and regular on the anterior slope, where there are 

 sometimes very obscure traces of about three radiating ridges. 



Length of the largest specimen seen, 0.67 inch; breadth of the same, 

 0.54 inch; height, 0.35 inch. 



The specimens show some variation in their proportions, as well as in 

 the regularity and distinctness of the undulations; the largest individual 

 from which the above measurements were taken being proportionally a 

 little wider and more elevated than some of the smaller ones, while its 

 undulations are less distinctly and regularly defined. As there are various 

 gradations, however, in these characters, I am at present inclined to regard 

 them as mere individual modifications of one species. 



Owing to the thinness of the shell, the undulations are often quite well 

 defined on internal casts, particularly along the anterior slope. 



The only North American recent species with which I am acquainted 

 that approaches this in size is the A. Newherryi, described by Dr. Lea, from 

 California. From this, the species under consideration differs in having its 

 apex nearer the posterior, and much more pointed and curved backward. 

 The undulations of its anterior slope also give the shell quite a different 

 appearance. 



Locality and imsition. — Fossil Hill, Kaw-soh Mountains, Nevada. 



