INVERTEBRATE PALAEONTOLOGY. 547 



In regard 1o the distribution of the species among the various groups 

 above mentioned, as well as other allied sections, whether viewed as genera 

 or subgenera, equally divergent views are entertained among high author- 

 ities, the same species being in some cases placed by the same author under 

 groups that he considers different genera. This is more especially the case 

 with regard to the distribution of American species by European authors, 

 as has been shown by Mr. Bland, of New York. 



Where such discordant views exist in regard to the classification of 

 existing species, of which not only perfect specimens of the shells, but even 

 where the living animals can be examined, it will be readily understood that 

 the study of imperfect fossil specimens is very far from satisfactory. In such 

 cases, we can only dispose of species provisionally, as may seem most prob- 

 ably correct, with the consciousness that a few additional collections in a 

 better state of preservation may, at any moment, show our classifications of 

 species in such groups to be faulty. 



The difficulty in distinguishing this from several allied groups (espe- 

 cially among fossil species) renders it very doubtful at what exact geolog- 

 ical period the genus Hyalina was introduced. I have the impression, how- 

 ever, that it occurs in the far West in beds belonging to the latest division 

 of the Cretaceous. It is certainly represented in Tertiary rocks, but seems 

 to attain its maximum development at the present time. The existing species 

 are rather widely distributed, being- found in Europe, North America, and 



the West Indies. 



Hyalina? occidentalis, M & H 



Plate 42, figs. 6, a, 6, c, d. 



Helix occidentatis, Meek and Hayden (1857), Proceed. Acad. Nat. Sci. Philad., IX, 135. 

 Helix Nebrascensis, Meek and Hayden (18G0), ib., 431. 



Shell depressed-orbicular; volutions three and a half to four, narrow 

 and convex above, prominently rounded or subangular around the superior 

 outer margin, and ventricose below; suture well defined; surface marked 

 with fine, regular, obscure, transverse lines, apparently not crossed by revolv- 

 ing strise; umbilicus deep, conical, and about half as wide as the under side 

 of the outer whorl at the aperture, which is subcircular or obliquely ovoid, 

 and moderately sinuous on the inner side for the reception of the penulti- 

 mate whorl. 



Greatest breadth, 0.33 inch; height, 0.21 inch; greatest diameter of 

 the aperture, 0.16 inch; breadth of same, 0.14 inch. 



